Phineas and Ferb/Characters/WMG

RE: Dr. Doofenshmirtz is Phineas or Candace's biological father
My theory of how it could be possible:

After the break up Doof and Charlene got married and had Vanessa but they divorced after her birth but before Candace's. Doof still shows that he has feelings for Linda (and vice-versa) and they marry and have Candace. When Phineas was born Doof probably built a baby carriageinator for Phineas, but he failed to do so and the machine probably blew up (or went berserk) and seemingly killed Doofenshmirtz (But it didn't). He was buried after the "funeral" but luckily he had his Dig up a Graveinator and dug it up. People don't remember that Doofenshmirtz was ever killed and believed he never did. Charlene still realizes he's still around so they both do the custody agreements on Vanessa. To this date, Doof never revealed any of this to this date, not even to Perry. And to make things stranger, Everyone in the Flynn-Fletcher family has seen Doofenshmirtz in the present day EXCEPT Linda. Coincidence? I say NAY! Even though he and Candace talked face to face, they probably don't remember each other because it's been too long or Doof pretended to not know her.

Stated in "Make Play", Candace is still 15 meaning that her and Vanessa are at leased a year apart since their birthdays are in summer (June for Vanessa and July for Candace as proven in D.W.G.T.B.B.T and C.L.H.H).

More evidence you say? Check it. Both the Flynn kids have at leased 1 trait Similar to Doof and Linda:

Phineas: Obliviousness from his mom. The ability to build stuff in a short amount of time (Like Doofensmirtz). Ferb probably gets it from his own mom but I don't want to go into that.....yet.

Candace: Craziness from her mom. Being able to survive things that normally kill a human (Just like Doofenshmirtz). Also if you notice, they are BOTH are very bad at planning which is why they usually tend to fail at their schemes.

Dr. Doofenshmirtz is Phineas or Candace's biological father
They both have triangle shaped heads and sprout hair, and they both build Homemade Inventions. Perry the Platypus seeks to occupy Doofenshmirtz so that he can never discover Phineas and tempt him to the ways of darkness. Additionally, Phineas once said "Soon there will be a giant rainbow that will sprawl across the ENTIRE Tri-State Area!" with copious hand-movements.
 * Not to mention they both spend copious amounts of time talking to a silent, green-haired best friend...
 * We see that Heinz and Linda have dated at least once in "What Do It Do?". Even though that particular date didn't go too well every relationship has ups and downs. It's completely possible that they have stayed together long enough for Linda to get pregnant. And then of course Linda met Lawrence and Doof met Charlene, and she might never even have told him that he had gotten her pregnant.
 * They were, however, pretty young, and he explicitly said 'And I never saw her again.'
 * Maybe it was their last date?
 * Or... Maybe Doof is changing the story for whatever reason.
 * He was definitely lying, how could he have known that Linda became Lindana? He obvious "saw her" at least once after that date:).
 * He likely just meant that he never saw her in person after that. Alternatively, someone might have simply informed Doof of this after the fact.
 * Two problems. First, Phineas is considerably younger than Vanessa, so any conception would have been well after Heinz met Charlene. Second, Heinz specifically mentioned that Linda became a pop star after their date. Again, this puts their date well before Candace's birth, let alone Phineas'
 * Vanessa is Candace and Phineas' half-sister? That would make a very interesting fanfic...
 * AND HOW!!

Also, Doofenshmirtz's ex-wife is always shown with things covering her eyes, but from what we've seen of her lips, face shape, etc., she looks exactly like Phineas' mom with a brown wig and tanner skin. And the only times she's shown are when the boys' mom is out for the day and Doofenshmirtz's daughter is trying to bust her dad. It's possible.
 * Considering the similarities Vanessa has with Candace, both physical and behavioral, this seems likely to be true. Unlikely that Disney will ever confirm it, though.
 * Perhaps Doofenshmirtz is Phineas' biological father. But Vanessa's mother can't be the same as Phineas' mother because they've met at a cooking class.
 * Unless we start involving Timey Wimey Balls, and given who Ferb's voice actor is. ..
 * This troper appreciates the Doctor Who reference, and feels less alone in the world because of it
 * Perhaps Doofenshmirtz isn't even aware that he has another child. He seems a devoted dad to Vanessa, if an embarrassing one, and so it's possible that his ex-wife just kept the birth of their son a secret.
 * It's possible Dr. Doof could be Candace's biological father - she and Vanessa are around the same age (if not exactly the same) - it's possible after having Vanessa with Charlene (his ex-wife's name, according to Dan Povenmire) and then eventually divorcing her, Doofenshmirtz could've had a brief fling with Linda that led to Candace's birth. Then they met again and along came Phineas.
 * Candace's three future children, Amanda, Xavier and Fred, have orange, yellow and brown hair. Candace has orange hair herself (and her mother's a redhead) while we can assume that the yellow hair came from Jeremy. But who could be responsible for Fred's brown hair considering that his mother's stepdad Lawrence is the only other brunet in the family and they are not biologically related? It would stand to reason for Candace's real father to have brown hair too.
 * A guess would say that Fred is Amanda and Xavier's step-brother.
 * Or, y'know, the brown hair came from *Jeremy's* side of the family.
 * That would imply Candace and Jeremy divorced. While such an occurrence is possible, considering how worked up Candace gets when she sees him walk by another girl, that idea just ruined about ten minutes of my day. I'm going with the theory that, because we never hear tell of another father, Candace and Phineas were made from donated sperm which unwittingly came from Doof. This also explains how he makes money for his inators. We never see him with a day job and he's far too incompetent to rob a bank.
 * Or maybe Jeremy died. Oh yeah, no need to thank me for saying that.
 * Except that it's been said several times that Doof's ex-wife pays him alimony.
 * Or he might be adopted.
 * Or Candice is a heterozygote for both orange and brown hair. Remember You Gotta Have Blue Hair.
 * maybe Fred and Xavier are very close cousins. The brown hair could still be a doofengene, though. Ferbnessa, anyone?
 * In one comic from the magazine, Candace said that she would always think of having 2.5 children when she's 26 years old, so that would imply that Fred isn't completely Candace's child, either from a guy other than Jeremy or a stepchild.
 * It's possible that Phineas and Candace aren't even full siblings. The closeness of the two halves of the family suggests that Linda wouldn't really make it a big deal. Also, Doofenshmirtz and Candace are both lactose intolerant.
 * Jossed - "What Do It Do" reveals that Doof did take Linda on a date once when she was a teen, and he never saw her again after that.
 * But that doesn't leave out Roger being Phineas' Dad, during the years that Mrs. Doofenshmirtz changed their name to Flynn out of embarrassment over Heinz--likely for some minor offense. Wait for the backstory.
 * But haven't you thought to yourself if Dr. Doofenshmirtz had Candace and Phineas in an illegal way, and he did 'that thing' to Linda while not noticing, as if he thought he was doing 'it' to his ex-wife?
 * This would also explain why Linda and Lawrence look so young in "Dude we're getting the band back together!" if Linda actually isn't all that old in the first place, and wasn't that old when she bore Candace and Phineas.
 * Doofenshmirtz could STILL be Phineas' biological father, by way of sperm donation. Linda wanted kids, she wasn't married, didn't want to do any of the hard work, and got the profile of someone who was a scientific genius, in the hopes that some of that would work out for her kid. Why would Doofenshmirtz be donating manseed? Well, he didn't always have that nice alimony check to create doomsday inventions.
 * This.
 * JOSSED FOR GOOD! Dan said himself in the interview that Dr. Doofenshmirtz isn't Phineas's biological father. This is the proof, but it's during the questioning session by the very end.
 * Fortunately (or unfortunately) as we've learned from some other incidents (the whole "Baljeet's last name" thing comes to mind) interviews/panels can't be trusted completely.
 * But that doesn't rule out the possibility that he's Candice's father.
 * He even said that the plot where Doof and Linda dated the first and last time was intended for the theory to be jossed, so I think it's more legit than the case of Baljeet's last name or any other cases.


 * The prevalence of Candace's Lactose Intolerance seems to be varying between episodes, but it IS a recessive gene, explaining how Candace could have it, but neither Vanessa or Phineas have it.
 * This is all well and good, but Dan and Jeff have insisted as recently as Comic-Con that they will not be introduced into the show, so unless they decide to change their mind, it will remain a WMG.
 * Well, I hope that they would at least give us hints on who their birth-parents were.
 * Won't they be introduced to in the movie then? Perhaps Across the Second Dimension will have an I am your father moment...
 * Nope, the missing parents didn't appear there. Maybe the theatrical movie?
 * This Troper and her brother always thought that Doofenshmirtz was Phineas' father and Major Monogram was Candace's. It explains their vendetta against each other, and Candace's penchant for busting Phineas' inventions...and their heads look the same.
 * In Make Play we see that Candace and the princess of Doof's country are almost identical twins. So Candace does have the Druselstein physical traits. Make of that what you will.
 * Fans say that Dr. Doofenshmirtz might be Phineas' father because of they both have triangle heads. Well, they're different type of triangle heads. Dr. Doofenshmirtz has an obvious looking chin, while Phineas doesn't have a visible chin, like what Baljeet had commented on before. It seems that Dan and Jeff are aware of this too, as no one who has a triangle head like Phineas appeared in the Doofenshmirtz Family Reunion.
 * This troper just likes to use Doof's Amnesianator as an excuse for all the contradictions in this theory because it makes such a great fic idea.

Doof and Linda were married/in a long-term relationship once, but both (or at least Doof) had their memories of their relationship wiped.
Except for their first date. It was possibly one of Doof's experiments or, more interestingly, the OWCA. They saw Phineas' potential even as a baby and wanted to avoid Doof raising him evil. Perry's main purpose is to ensure the memories stay repressed/scrambled and Phineas and Doof never meet.

Vanessa's mom is Phineas and Candace's mom, living a double life for reasons unknown.
Maybe she's also a secret agent, or she works for the government.
 * Jossed: Linda Flynn and Charlene Doofenshmitrz are in the same cooking class, and commiserate when they both get calls from their 'crazy' daughters at the same time.
 * No, that doesn't mean it's Jossed. It means that the person who is both Linda and Charlene is really a Time Lord. One is a regeneration of the other (which one's older/younger is up for debate).
 * No. You can't explain everything away by saying that he or she is actually a Time Lord/Lady. Some things just don't make sense.
 * Though it could explain some things in other shows and movies...
 * It could be that Linda has multiple personas and she talking to Charlene is a whole metaphor. It also explains where Charlene gets all the money to finance Doof and also where P&F get their money, Lindana made a great fortune which is in use actively by the shows top three mad scientists.
 * They're also both Tyler Durden.
 * Linda and Charlene were college roommates. Linda may have introduced Heinz to Charlene.

Charlene is Ferb's mother
...wait a minute. That's mean Ferb and Vanessa are...Squick

Ferb's biological mother is French.
Well, you explain why he apparently celebrates Bastille Day, then!
 * This makes sense, seeing as he seems fairly fluent in French in the runway episode.
 * Ferb's grandparents don't like their daughter-in-law because of the traditional British-French rivalry. This continued animosity lead to Ferb's parents getting a divorce.
 * This needs to be canon.
 * I truly am in love with this theory, and whoever came up with it should be a writer, because it certainly should be Canon.
 * If she's also an alien, she might pose as a French citizen. Also, if Lawrence/Max Modem's song "Alien Heart" is based on her, it means that the phrase "girl out of town" has a double meaning.

Ferb's mother had French-American dual citizenship and was living in Great Britain, and he was born during a visit to the American Embassy in London.
Or, assuming it hasn't been Jossed already, while staying at the American Embassy in some war-stricken country while his mum was working to win that Pulitzer. Hey, it could happen.

Candace and Phineas's biological father is Greek, or has a great interest to Greece.
Candace and Phineas's name are of Greek origin. Perhaps their biological father named them that way.
 * Or Linda may have been interested in Greek things.

Doofenshmirtz is Phinea's Biological father, but Charlene is his real mother
Linda does not have the triangle shapped head. Candace has the shape of her face similar to Linda's one. So Candace is the daughter of Linda. Because of the theory of Doofenshmirtz + Linda is jossed, it's probable that Doofenshmirtz have the child with Charlene and in some way ended on the Flynn house. (e.g. Linda cannot have children after Candace, so her husband managed to "get" a child from somewhere)
 * I presume you mean "the daughter of Linda"?
 * If Charlene is Phineas' mother, then where did he get his red hair?
 * Corrected. Maybe the red hair could be a genetic mistake or something.
 * Perhaps Linda IS either the egg donor, or the DNA donor. Thanks to recent advancements in medical science, Charlene could have donated an egg, Linda could have had one of her eggs fertilized by a random sperm donor (Doofenshmirtz in this case) (after all, we don't even know if Linda and Candace's dad were married), and Charlene carried Phineas to term.

If above theory is correct, than Charlene was once a redhead
She just dyed it black to relate to her goth daughter without embarrassing her.

Phineas real biological father is Pyramid head
Which explains why his head is triangle shaped.
 * From here?

Perry was Phineas and Candace's bioligical father transformed into a platypus.
A theory developed by the wiki. Perry (with a different name) married Linda and had Candace and Phineas. They later got divorced since he was always gone (in his agent missions). He then had an accident and transferred him into a body of a platypus. Things like this are supported by the episode "Phineas and Ferb Got Busted", since there were flashbacks of Phineas and Ferb's childhood (before Perry arrived).
 * All the other agents might have be involved in that same accident. All the agents participated in the same mission of great importance. Perry was probably held hostage (perhaps by Doofenshmirtz) and the agents arrive to get him out, which was a trap and they have all been transformed.
 * Let's make a simpler WMG. See the "Perry isn't an ordinary platypus" WMG below.
 * Come to think of it, this is a good explanation as to how Candace-2 is in the movie. And also how, even without any training at all, Candace-1 is able to wrestle an alligator into submission. In the water, no less. FOR CONCERT TICKETS. Clearly such skill is genetic.
 * Especially athletic skills.
 * If this is true, then the episode "I Was a Middle-Aged Robot" where Perry spends time with Candace in a father-daughter event and even cheats to make Candace win is a Crowning Moment of Heartwarming.

Linda had Candace and Phineas through sperm donation from the same person.
In Phineas and Ferb Get Busted, during the Little Brothers song you see Linda showing Phineas to Candace. Note that no father is seen with her. Seeing how Candace and Phineas never even mention another dad, and refer to their stepdad as "Dad", it would just make sense for them to have been sperm donated.
 * Note that this idea was based off of Doofenshmirtz being a sperm donator (see Doofenshmirtz is Phineas' biological father). As much as that makes sense, the above is simply stating what it does.

Candace was heavily abused mentally by her biological father.
This would easily explain her incredibly neurotic, extreme OCD, and sometimes borderline psychotic behavior. Without even laying a hand on her, her father, Kevin, inflicted more pain and suffering than anyone else would physically or sexually. After Phineas was born, Kevin was starting to do this to Phineas and Candace had the courage to finally bust her father for what he was going to do to Phineas and what he did to her. Linda divorced him right then and there and the US Government gave EVERYTHING to Linda after hearing what Kevin did.

The reason we never hear anything about Phineas and Candace's biological father was that his abuse was SO bad Linda never mentions him again and Candace completely forgot him after he was gone. The zebra she see in her hallucinations calling her Kevin is actually calling her by her biological father's name and is probably a representation of one of the stuff toys Kevin used to torture her. And Candace's obsession with busting her brothers is her own way of giving herself any sort of self-worth.

Phineas and Candace's dad is a well-known musical artist.
He and Lindana had one of those two-second celebrity marriages. This theory might add some Fridge Brilliance to the Max Modem episode--their dad is insecure because he knows that their mom was with a big star before she met him. As with the demigod theory above, we may guess that Phineas and Candace inherited some musical ability from their father as well as their mother.
 * Related to the theory that the missing dad's voiced by Danny Jacob, all songs sung by him are actually sung by the dad a long time ago. Those songs coincidentally fits the experiences Phineas and Ferb were going through.

Norm is Phineas' biological father.
Doof didn't build Norm. He found a man off the street and turned him into a Robot to save him the trouble of making one. That man just happened to be Phineas' real dad. He also isn't a very nice Man as he wants Phineas and Ferb to get busted by Candace so he tried to destroy Perry so Doof's inventions will never get rid of Phineas and Ferb's stuff But he lost some IQ points during his Robotifacation and now doesn't remember that he was ever Human and voluntary works for Doof. The robot in "Phineas and Ferb Busters" is Norm's Brother or something.

Thaddeus (and most likely Mandy, and probably Thor too) is Phineas and Candace's cousin from their biological father's side.
Thaddeus being one of the only characters in the series with the same head as Phineas and being a super-inventor (but not as much as Phineas) convinced this troper. They didn't know about their biological father's extended family which explains why they didn't knew each other (If this troper could recall).

Phineas and Candace's real father is a major Workaholic.
He works outside the house so much that Phineas and Candace were unable to recognize him, thus calling their step-father Lawrence "Dad". As for why he could disappear? He probably died while he worked.

Phineas and Candace's biological father is Roger Doofenshmirtz
Linda became a millionaire thanks to her music career, but she really wanted children and couldn't find a suitable mate until Lawrence (She tried with Dr. Doof with nefarious consequences and decided not to try for a looong time). So she went for sperm donation.

Level 100-size ego Roger Doofenshmirtz was of course a prolific sperm donor. His profile said "Very charming pillar of the community" or something like that. So, Linda went for it, twice.

Roger knows that Phineas and Candace are his children. Picking Candace as mayor for a day was a Batman Gambit to meet her, too bad the space continuum had to change.

Phineas And Candace's Father Was A Construction Worker/Architect
And He Died On A Construction Site. Linda, Crushed by the News. Became Worried That Phineas Would Take After His Father. And, After Watching Phineas (and Ferb) For 10 Years, Finally Calmed Down (Hence Why She Doubts That Phineas (And Ferb) Could Build "These Amazing Things") This Would Explain Why, When Confronted By Evidence, She Begins To Freak Out And Punishes Them.

Phineas's Father is a mad scientist
It would make sence, consitering Phineas's inventing skill. However, this mad scientist was not Doofenshmirz, but rather another mad scientist.
 * Perhaps Professor Kevin Destructicon is his father and Candace's father (I just came up with that because of the zebra that calls Candace Kevin).

Heinz Doofenshmirtz is Ferb's Mother
I don't know, I just felt like their needed to be another ridiculous theory on here.

If Perry's Candace and Phineas's real father and the above theory is correct, then Perry used to be evil.
The O.W.C.A. decided to use some animal ray to change him into an animal and trained him to do good.

Phineas's biological father won't resemble Phineas.
It'll be Phineas's biological grandfather.

Ferb isn't an alien.
He's half alien. The Max Modem song was written about her, not Linda. I mean there are two genes that control hair color, it seems unlikely that Ferb would just so happen to have enough genetic mutation to create a third. As for where she is now? Dead, maybe. Perhaps her planet needed her and she died on the way back to her home planet.

Phineas and Candace's dad will be voiced by Danny Jacob.
What? He quite sounds like Phineas himself, plus it would be awesome if there's a scene that before he died, he said that he'll always be with them, which makes it quite literal since we've heard Danny Jacob singing Once an Episode.

Phineas was blamed by Candace for the loss of their biological father and was mentally traumatized by it.
Whether or not their father divorced Linda or died, Candace felt extremely upset at both losing him and seeing her mother unhappy, and on instinct, blamed Phineas for it, saying Linda and their father would still be together and their mother would still be happy if Phineas wasn't there. Phineas believed her and was in extreme guilt for most of his childhood until Linda remarried and he met Ferb.
 * Both of Phineas' depressions in "Summer Belongs to You" and "Christmas Vacation" were triggered by Candace yelling at him and blaming him for their problems. This caused Phineas to trigger memories of his early childhood and become mildly depressed.
 * To sum everything up, Phineas is and always will be an enthusiastic, happy child, but when Candace blames him for something, it throws him into a pit of despair. (This may also happen if Phineas' biological father was ever mentioned.)
 * That's....Nuts. Awesome!

Phineas and Candace's real dad and Ferb's real mom will appear in A Phineas and Ferb Family Christmas
They did mention that there will be very special guests.
 * Let's say that it's Jossed before the episode actually started.

Candace's biological father is Doof, Phineas's is MacGyver, And Ferb's is The Rani
The Candace has been explained already. Macgyver makes amazing things our ordinary things. Phineas warps reality with average tools. Ferb's is besause only a Time-Lord he can be that smart, is possible to Timey-Wimey Ball, and is not evil because Rousseau Was Right.

Ferb killed Phineas and Candace's biological father.
And stole his voice.

Phineas's biological father is one of theSpy vs. Spyspies
They have the same head shape and the spies are always building weird weapons.

Ferb's mother is an alien.
Candace was actually half-right. Not only does it seem unusual for her to be wrong about something related to her brothers, it would explain quite a lot. After all, Ferb is the only character with an unnatural hair color, he can defy the laws of physics by moonwalking up a tree backward, he speaks Martian, and he's familiar enough with alien tech to modify Meap's ship (not to mention he apparently knows what is and isn't "street legal" for spaceships). Most likely his mother was a Human Alien who died or was forced to return to her home planet and can never return or contact her Earth family again.

Candace watched Ducky Momo to get over what happened to her and Phineas's father.
When you see Candace's flashback for when she became obsessed with Ducky Momo in Nerds of A Feather, she looked like she was upset/about to cry until the TV turned on. Maybe her parents were arguing/ mother was crying about their father death or something else (since we don't know what happened to their dad) and Ducky Momo was her only way to deal with it, even later in life. The reason Phineas might not be affected is because he was too young to remember.

Phineas and Ferb one time invented a machine that warped the laws of probability.
The series has so many Contrived Coincidences, it has to be either this, or some omnipotent being really likes to mess with people. Plus, they've built stranger things.
 * Hence, they invented "The Infinite Improbability Drive".
 * So in other words, either P&F have a literal Deus Ex Machina or God just loves to screw with the citizens of Danville?
 * God must love to screw around Candace.

Phineas and Ferb are pre-Christmas elves, living like everyone of us.
Supporting this Guess is the theory that Santa's elves are bred from actual human beings, and after they die, are selected by Santa to be recruited into the factory on the North Pole judging from their talents and skills they had in life. Phineas and Ferb can invent ridiculously involved electronic and digital contraptions in mere hours, can host concerts and other public events within a day and still get a sold-out area for it, and never complain a bit about the labor! Once they die, they'll be shrunken down to a foot high, and hired to work for Santa. The events seen in the Christmas special foreshadows all this, which also explains why Santa is so taken by Phineas' letter.

Phineas and Ferb ship each other with Isabella
Phineas ship Is with Ferb and Ferb ships her with Phineas.

Phineas and Ferb made a new Time Machine and will use it at the end of Summer.
After the episode where they built the time machine, Phineas realized that it could be used to keep Summer going forever and built a new one. After 104 episodes, they'll just use it to go back to the beginning of Summer and make new inventions. They can also use it to go forward in time, which is why they had the Christmas Special. They just fast-forwarded time so that they could celebrate christmas.

Phineas and Ferb are just kids with an over-active imagination
The stuff in Phineas and Ferb are just them playing games. Their friends (Isabella, Baljeet etc) are their best friends who also enjoy to play these games of pretend with them. Cadence, not showing it, is just their loving Sister, who plays along with them, because she loves them. But sometimes takes it too far and it seems like she actually means it. She just plays the role of the villain in their games, and everyone knows it. Her Boyfriend, Jeremy, does love Cadence and respects Phineas and Ferb, so he plays along too. Perry is not really a Secret Agent. He's just a pet Platypus. He dosent do very much, like they always say. Their neighbour, Mr.Heinz Doofenshmirtz, is an extremely nice person, and Phineas and Ferb can't see him as evil. Perry will occasionally go to Heinz's house by curiosity, like most pets do, therefore, Phineas and Ferb got the idea to make Perry a Secret Agent with the "evil" Dr.Doofenshmirtz as his "arch-nemesis" in their games.

Phineas and Ferb don't do the crazy things the show depicts them doing
Instead, 99% of the show's events is Candace's delusion. This is why, whenever she drags her mom to see what they've done, it's all magically vanished. She has a lively imagination which, unfortunately, chose to go out of hand the summer Ferb visited. A little longer, and she'll be in a mental institution.
 * This is supported by Candace's zebra hallucinations. Perhaps she keeps her fantasies in check for the most part except for what she imagines Phineas and Ferb doing - and when she doesn't, she loses all touch with reality. That would explain her hallucination in the episode where she didn't touch the hallucinogenic moss.
 * Though there is something to be said for the placebo effect
 * This would also explain the horrendous physics in the show - even compared to other cartoons. Hallucinations wouldn't require any logic.
 * Also, Agent P is actually Candace's super-ego, which would explain why they seem to be connected (see below), and the adventures P has every episode are actually Candace's power fantasies and how she tries to justify her delusions disappearing to herself. And in 'reality', Doofenshmirtz is actually Candace's therapist (hence why he likes talking about miserable childhoods so much). 'The Cabinet of Dr Doofenshmirtz', anyone?
 * Candace has a crush on her therapist, thus explaining any... 'awkward' moments between Perry and Doof.

Epcot exsists because Phineas and Ferb built it
They went on the "Its A Small World" ride and thought they can do better. Besides, the whole "Innovations" area screams of their handiwork.
 * How could they? Epcot probably opened when their mom was Lindana.
 * Time Travel?
 * Or Phineas's biological father.

Phineas and Ferb are the reincarnated forms of Doofenshmirtz and Perry
On the Star-crossed lovers page and the Twincest page, it is mentioned that star-crossed lovers that commit dual-suicide get reincarnated as twins. Well, Doof and Perry don't seem to be headed that way anytime soon. Perhaps they die sometime after the proposed future of "Quantum Boogaloo" (unless, of course, it didn't happen) and are reincarnated in happier times. After all, Phineas and Ferb aren't twins, either.

Phineas and Ferb need each other to do all that stuff.
Ferb does most of the building. Without Ferb, Phineas would be just another hyperactive kid drawing pictures of all the awesome stuff he'd do if he could...and without Phineas, Ferb would have just sat under that tree reading that book all summer. I'm pretty sure people have thought of this, even though it's never been directly addressed in the series...heartwarming, isn't it?
 * That's... basically canon.

Phineas and Ferb were never hit by the dull-and-boring inator nor the dynamic-inator.
If you pay attention when Perry fires the dynamic-inator, it does not hit P&F, it is confirmed later that it did hit Linda. Instead, Phineas seems to *recover* when Isabella begs him to be creative. Their original plan was "I know what we are going to do today! Not be creative" and was just yet another attempt to do something they didn't do before. But Izzie was able to finally convince Phineas that being creative is the best they do and one day without being creative was not worth it.

Phineas and Ferb discovered something that doesn't exist the day after "Hail Doofania!"
Phineas' last line implies that they're going to search for a unicorn tomorrow. The closest thing they can find is the tortoise-frog-unicorn crossbreed shown in the opening credits, which doesn't exist.

Phineas and Ferb Have Everyone Under Mass Hypnosis
They never built any of that stuff. Candace can never show her mom because around the same time every day, the trance she was under gets broken. The boys just put everyone under hypnosis and made them THINK they were doing or seeing certain things. Perry is even an unfortunate victim and thinks that he and Doof are responsible for all of the stuff disappearing.

Phineas and Ferb built a rocket to Triton
The opening credits shows them on a celestial body with tall rock formations, wispy atmosphere, and a large planet with blue and purple cloud bands in the sky. Triton is the only body in the Solar System that fits the bill. The planet in the sky about has to be a gas giant, and its coloration looks much like the standard depiction of Neptune. No Neptunian satellites are known to have any kind of atmosphere except for Triton, which has geysers that provide a sort of localized temporary atmosphere.

Phineas takes anti-depressants
Doesn't anyone else find it creepy that Phineas is happy all the time and never disappointed?
 * Hmm, I don't know about Phineas, but maybe Charlotte...
 * I don't want to speculate on what Isabella is on, but it's something strong. Really strong.
 * She's high on love!
 * This thread is funny, because I always thought their dad was on horse tranquilizers.
 * Maybe Ferb is taking anti-depressants so he can stand being Phineas' slave, and Phineas controls Ferb with an invention.
 * Can't we just say that Phineas is just happy?
 * Sure we could. But what fun would that be?
 * Exactly. Wild Mass Guessing, of course.
 * Nah, Phineas just suffers from the rare, undocumented neurosis that is the opposite of clinical depression but not mania either.
 * You mean hypomania (which is certainly not undocumented)? Considering Candace's hallucinations and how excitable she is, it could be genetic, and shows in Candace as some full-blown manic disorder.
 * He was unhappy when he found out that he couldn't pull off "street."
 * Yeah, he isn't ALWAYS happy. Part of his daily summer ideas come from being unsatisfied with something at that moment, such as complaining about how the portions at high-class restaurants are so small. Plus, you can't forgot his bail-out of a music deal when becoming a one-hit wonder! But most of the time, he IS deliriously happy.
 * He's on a constant sugar high. (yes, it's quite possible to spend large periods of your childhood on a constant sugar high, this troper did)
 * Supported because Linda makes sugary treats almost every episode.
 * Phineas' parents' passive, almost neglectful parenting and the fact that they are a white family in a wealthy suburban environment makes be believe that they would definitely put him on some kind of pill, even if he didn't truly need it. Anti-depressants or the like is a very high possibility to me. Maybe both Phineas and Ferb are on them?
 * That's Racist!
 * Or maybe he's a Stepford Smiler
 * Well, we do see him depressed in Summer Belongs to You.
 * Yet that was near the end of 40 hours journey. It is consistent with the pills theory (the effect of the anti-depressants wore off).
 * But don't forget his complaining earlier about how They've hardly done anything
 * You guys clearly haven't taken anti-depressants ever. They does not make one happy all the time, they just keep you from being horribly depressed all the time.
 * Which explains how he's never disappointed...but why is he always happy?
 * I'm sure there's a name for this disorder but he has a disorder in which his emotions are extreme. He's never happy or sad. He's wonderful or depressed. His family couldn't stand seeing him depressed most of the time, so they gave him anti-depressants. Now all that's left is the extra happy.
 * Bipolar Disorder... however, you give someone with Bipolar Disorder anti-depressants and it just pushes them more manic.
 * He may have taken them after the pilot "Rollercoaster" since he started out as irritable and sarcastic before Characterization Marches On.

Phineas started going through Puberty in "The Beak"
Because, as much as I love Phinbella, I found it random that Phineas would give something more than a subtle hint on his affection with Isabella.
 * That would be the real reason why his voice was changing when he was "The Beak"?

Phineas is aware of Isabella's feelings for him.
In fact, he feels the same way. He just wants her to figure out this fact for herself. He drags Ferb along with him, because he also aware that Ferb has a crush on her too, and he wonders what's a threesome is like.
 * But isn't he a little young to know what a threesome is?
 * Ferb taugh-er, I mean, yes. Yes he is.
 * "Hey Ferb! I know who we're gonna do today!"
 * just need to say this made me laugh like an idiot for about three minutes
 * That's actually pretty plausible (minus the threesome part) in light of "The Chronicles of Meap." At the end he says he had Isabella's cuteness factored into the cuteness detector from the beginning. Except at the beginning of the episode he tells Isabella in unambiguous terms that he doesn't know anything or anyone cuter than Meap. So it looks like he finds Isabella cute, and knows that she wants him to notice her cuteness, but is reluctant to admit this. That or the writers goofed.
 * Another thing that supports this theory is the end of the movie when Isabella kisses him. It basically forces him to momentarily show his true feelings for her as shown when he tries to stop the amnesia-inator from firing.
 * It seemed more like Phineas really didn't know Isabella liked him, though his reaction seems to indicate he does like her.
 * My impression is that he doesn't feel ready to commit to a relationship yet or thinks having a girlfriend will interfere with the daily activities. He always invites her to join in as compensation.

Phineas has low-self esteem towards the idea of any girl be interested in him romantically.
Phineas has shown that he is a Shipper on Deck for both Candace/Jeremy and Ferb/Vanessa, but is completely oblivious to Isabella's own feelings for him. The reason for this is that he has trouble accepting the possibility that there might a girl out there who's actually interested in him. To add on to the theory, Phineas has a crush on Isabella but is completely oblivious to Isabella's own crush on him.

Phineas is a closet Nightmare Fetishist
He seems too happy with disturbing imagery.

Phineas might have a crush on Stacy.
If you look at " Put That Putter Away!" during "Disco Miniature Golfing Queen" it certainly seems like it.
 * Some time after reading this WMG, I watched Disco miniature Golfing Queen and in just about every Phineas-Stacy interaction, Phineas seemed very interested in her. Perhaps some sort of confirmation bias though.
 * This troper, who admits to crushing on Stacy, thinks Phineas better watch out...
 * "Put That Putter Away!" was one of the first episodes I ever watched, and I got the same impression. But then I watched more episodes and realized that Phineas gives that look to anyone who's dancing or singing with/in the vicinity of him. Including Ferb.

The whole series is Phineas' dream while trapped in a coma
Simlar to the series is Dr.Doofenshmirtz's drug abuse WMG above, Phineas was caught in some accidnet on the final day of school that left him in a coma. Ferb, his Parents, Candance, and his friends all exist, but their increadibly exaggerated in his dream. He never owned a platypus, instead Perry is his favorite stuffed animal from his childhood that his mother put in his hospital room to liven things up. Dr. Doofenshmirtz is one of the doctors that keeps care of Phineas, the other doctor is Dr. Monogram who in his dream is Major Monogram, he and Dr. Doof have a small grudge since medical school hence their fighting in his dream. Carl on the other hand is a male nurse that visits his room often.
 * That actually sounds really heartwarming. A little sad, but heartwarming none the less.
 * Several people do think Doof is a pharmacist at first glance.
 * OR: the series is a chain of Phineas' psychotic hallucinations and delusions. Why is Phineas always happy? He's ON FREAKIN' ANTIPSYCHOTICS! Candace's fixation with busting him is his brain trying to snap him back to reality. Isabella represents his sexual frustrations and fantasies, Perry is his superego (saving the world is pretty fantastic), Dr. D is his id (some people just want to watch the Tri-Stae area burn) and Ferb is his ego (level-headed). The activities the boys get up to represent wishful thinking and regret: in the real world, Phineas may not have spent much time with erb and mayhbe even despised him. The escapades serves as a mechanism to ease the guilt.

Phineas is the Anti-Christ.
The Anti-Anti-Christ, at least, explaining his blatant Reality Warper abilities. Danville is Strictly Formula and he keeps it that way (albeit unconsciously) because that's the way he likes it. However, as he becomes disillusioned by his growing awareness of humanity's imperfections, he resolves to reshape the world, and his friends find themselves dragged into something bigger than any of them ever imagined. And then Isabella defeats the Anthropomorphic Personification of War in combat.

Phineas is slowly revealing his dark side.
Towards the end of the "Summer Belongs to You" special, just as Candance begins to protest riding the giant paper airplane, he angrily snaps at her in a very un-Phineas tone. This could be the start of Phineas turning evil or worse, also during the music number in Paris, phineas pulls off some mad scientist poses while messing with pinwheels.
 * This troper for one does not want to think about the notion of demon path Phineas.

Phineas is in school.
The whole show is just his class doodles, sense he wishes he wasn't there. Thus explaining the characters just looking like shapes, the beyond the impossible stunts, and Perry's 'secret life.' Phineas is just very, VERY bored in school and has a wild imagination.

Phineas is the Anthropomorphic Personification of imagination and creativity reincarnated in a human form.
Ferb is his assistant. Nobody except maybe Ferb knows because Phineas lost his memories when he was born. This is why he and Ferb are not limited by the laws of physics.

Phineas is a lot less cheery during the school year.
The standardized learning environment gets to him, making him cranky and irritable. That's why he was more annoying in the first episode--it was only the first day of summer vacation, so he was still detoxing from all the stress.
 * Seconded. I've always thought this.

Phineas is impatient (and possibly clumsy)
Which explains why Phineas creates things really fast, and he quickly gives up after he only found out in a rush that there were no materials left when Phineas and co. were stranded in an island in Summer Belongs to You and the fact that he yells when Candace doesn't want to ride a tricycle.

The series is just Phineas' daydream.
Yes, yes, I'm sure this has been said before, but think about it this way. The first episode, of course, starts with the theme song (as does every other episode) and the theme begins with P&F sitting beneath the tree in their backyard, Ferb reading and Phineas staring off into space. The camera zooms in on Phineas to show what he's thinking. We never see them getting up from that tree or see Phineas' daydream cease, therefore everything from that point on is in Phineas' mind (the rest of the theme song and all the episodes). The series will end with Perry chattering, startling Phineas and bringing him back to reality, revealing that the entire series has actually only lasted about five minutes and that it's still the first day of summer. He'll then proceed to ask Ferb what he wants to do today.

Phineas is a Sociopath
His whole "sweet, innocent optimist" bit is all a facade. He knows what he does makes Candace upset but he's incapable of empathizing with her. And he knows about Isabella's crush on him, he just doesn't care. Everything he does is for his and possibly Ferb's gain, but once his brother gets in the way he'll be "off to kindly Old Man Simmon's farm" with Bucky.

Phineas was aware of his sudden personality change.
The writers picked him apart, coming up with just the perfect personality for the show...and he was helpless to stop it...of course, by the time they finished, his personality was so mellow he didn't really seem to mind. It made all the other characters mildly uncomfortable the first time they met this new Phineas, but in the end, they accepted him as a fellow cast member.
 * laughs maniacally*

Phineas isn't just Oblivious to Love - he's oblivious to how normal people express it.
Think about it. Phineas basically has no hint of subtlety anywhere in his personality. Every time he had to help someone else with romance, his first plan was always one of his elaborate schemes. It's always a huge concert or a tropical love cruise, or something of that nature. It's not impossible that he thinks that's how everyone demonstrates romantic attachment. So even Isabella's most transparent hints are still orders of magnitude more subtle than anything he'd do - which is why he doesn't notice them!

Phineas knew that Candace and Linda would bust them if she saw what they were doing in that episode...
He didn't want to be busted and not build inventions anymore, so he faked it and told Ferb. Isabella, Buford, or Baljeet never knew his plan, and Phineas got too engrossed in it that he did it in front of Candace. When he was reminded, he wanted to build again.

Ferb has autism/Asperger's Syndrome.
This is just something random that occurred to me, but really, doesn't it make sense? It would explain both why he talks to rarely and how he manages to be so smart and good at building things (he's an autistic savant).
 * He also doesn't really make eye contact. I think you're on to something.
 * Actually, I just see Ferb as the Teller to Phineas' Penn.

Ferb knows about Perry's double life as a secret agent
In fact, he discovered it on the day he was first became a secret agent. However, in order to not get his mind wiped, he took a secret vow of silence. But, it's not very strong, so he says a few things so they won't suspect something.
 * Why you think he was the one that exposed Perry in his dream?

Ferb has Marfan Syndrome
It would explain why he has such abnormal long legs, as shown in "A Hard Day's Knight". Although I don't think that he should be jousting if he does. Considering his physique, Doof is also a good candidate for having Marfan's.
 * See the Nelson Textbook of Pediatrics 17th Edition, pages 2097, 2325, 2328, 2332 and 2339. Some details of Ferb are quite consitents with the Marfan Syndrome abnormalities.

Every girl in Danville except Adyson, Isabella, and Ginger has the hots for Ferb
Who wouldn't?
 * No, I don't know what you mean.
 * She likes Baljeet.
 * No, that's Ginger. What about Adyson?
 * I'm assuming the OP made the rather offensive assumption that because Adyson is the most "butch" of the girls, that automatically makes her a lesbian.
 * Every girl except those three? What about Candace?
 * Every female child?

Ferb has a goth fetish
Which would explained partly why he has a crush on Vanessa, besides the fact that she's the hottest character in the show, and its a voice actor injoke.
 * Jossed. Sort of... She's a punk, not a goth.

Ferb or one of his relatives is somehow connected to the (singing) Doofenshmirtz Evil Incorporated!!!!! building.
I mean, look at it. And the head shape looks to be hereditary. Though they probably don't have direct connections with Doofenshmirtz, since the building existed before Doof bought it.

Ferb told Phineas about his crush on Vanessa during "Spa Day."
After switching from another subplot, we catch Phineas and Ferb at the end of a conversation where Phineas laughs and tells Ferb his "secret is safe" with him. Later, in "Summer Belongs to You," Phineas' "boy and girl in the City of Love" comment implies he's a Shipper on Deck for Ferbnessa, so it's possible.

Ferb is half-Martian
This would explain who taught him the language, why his hair is green, and where he got his musical ability (every part of a Martian's body is a musical instrument, for crying out loud!)
 * That part of "Unfair Science Fair Redux" is probably a parody of the serial Flash Gordon Conquers the Universe, where Professor Zarkoff can randomly speak the language of primitive superstitious aliens, which is actually reversed tape of the lines in English. If we follow that parody, then Ferb's knowledge of Martian language is because the Martians' more advanced ancestors colonized part of Earth, died out, but passed their language on to a Native American tribe that Ferb coincidentally studied.

Ferb is short for Fabian.
The Fabian Society is a British socialist group, and Lawrence may have named his son after them. Fabian Forte the pop star is another possible source. Phineas was very young when he and his stepbrother met, and his mispronunciation of the name may have led to a family nickname which stuck from then on.
 * The fact that a real-lfe set designer named Frank, nicknamed "Ferb" by his wife, it the character's namesake doesn't have to carry over to the show. Candice doesn't remember what "Ferb" stands for. If it was such a common name it would more likely have stuck in her mind.
 * Candace couldn't remember the word aglet even after she witnessed a Crowning Music of Awesome performance that included multiple spellings of the word...
 * Its short for Ferbanacci.

"Ferb" is short for "Frederick."
Years from now, Ferb and Vanessa will get married, have a child who sort of resembles Vanessa's dad, and name him after Ferb, but call him Fred for short. With both parents busy in the secret service, young Fred will spend his summers with his aunt Candace and his cousins Xavier and Amanda...

Ferb's full name is either weird or unpronounceable.
Really, if he's a half alien (from the trope above), his alien mother would give him a weird name.

Ferb doesn't talk because most people can't help reacting strongly
More often than not, his few lines get a fairly wide response-- Cheer Up Candace had the airline food joke, for instance, and the Lizard Whisperer his speech, both of which garnered a very positive reaction from a large crowd. The only one who really seems capable of ignoring him is Phineas (and, sometimes, Candace), as evidenced by Ready for the Bettys; that may be why there are so many implied conversations between the two that the audience never witnesses.
 * In the Lake Nose Monster, it's shown that he gets a big reaction even when people don't like what he's saying: "Hey, everyone! That British kid is saying something really, really boring!"

Ferb never got hit with the dull-and-boring-inator.
He just kind of went with what Phineas was doing.

Ferb actually is short for "Oh. Here it is."
...For some reason. It was only a coincidence that he found the Pizzazium at the same time he was telling Vanessa.

Ferb is a changeling.
Think about it; he's the only one in the P and F universe that has that hair color, he doesn't act like anybody else, he can commune very well with animals, and in the supercomputer episode, his thought pattern was that psychadelic rainbow. Also, it could help explain why he can build those things in insane amounts of time. (For those who don't know: a changeling is a fairy baby switched at birth for a human infant by its parents, and the human parents are almost never any the wiser.)
 * I don't know... Ferb looks obviously like his father, especially his grandfather, a sign that the father (Lawrence) is Ferb's biological father.
 * True...But can't faeries shapeshift? ((Would also possibly explain the muscle thing from "No More Bunny Business.))
 * My theory is that he's a faerie and nobody knows it but his side of the family, and they've all learned to accept it by now. I think that he could have been bargained for (because if you give faeries what they want, they'll give you what YOU want, although it's at a price) and that he was sort of arranged so that he looked like his dad. He doesn't know he's a changeling and nobody will ever tell him unless another faerie comes back to get him.

Ferb (or FERB?) is an acronym of many names.
Anyone noticed that "FERB" in the title is uppercase instead of "Phineas" with lowercase letters? Plus Ferb did say that his nickname is short for something.

Candace was meant to be a boy
The Zebra she often hallucinates always calls her Kevin. Perhaps she was meant to be a boy, but instead was born Candace. The Zebra is a slider or time-traveler who is trying to correct the time-line; but the most he can do is disturb Candace once in a while because he lives in her brain. (It's Mental Time Travel.) The Zebra could also be why Candace thinks Suzy is evil. The Zebra doesn't want Candace to do anything that would be awkward when it's translated to the corrected time-line.
 * So Candace and Jeremy weren't meant to be?
 * Maybe he's gay.
 * He did seem pretty interested in DD
 * Or the entire cast is gender flipped and slightly renamed to avoid embarrassing the actual family.
 * So Kevin wants to bust his little sisters, Vanessa and Vern (short for Veronica), because their inventive activities aren't ladylike.
 * Or Kevin Flynn is a flatchested cross dresser?
 * Kevin Flynn is a computer programmer.

Candace is a diclonius.
Or rather, part-diclonius, like the guy with the wig. Whenever her hair gets ruffled, the nubs of flesh-covered horn stick out from her hair. She can't use the arms because she's not a full diclonius. Phineas may have been affected by the dioclonius genes too, but they somehow merged into one large horn sticking out the top of his head.

Bad Future Candace stops Candace from busting Phineas and Ferb in EVERY episode
Before she stopped Linda from seeing the rollercoaster, she revisited all of her brothers' adventures and pulled all the strings behind the scenes, making it look like Contrived Coincidence so Linda would never see anything.
 * Possible. You can see BOTH future-Candaces in Roller-coaster; the musical.

Candace has severe OCD
Let's face it, it would explain a lot about her behavior. Hell, it would explain almost everything about her behavior.

Candace is full of teratomas.
She has an eye in her central body cavity (as well as internal bleeding that doesn't seem to be out of the ordinary). Need I say more?
 * More then likely, it's not an eyeball, but that thing in the middle of nervous system, whatever it's called.
 * Maybe it's just something that she ate. The eye "moved" because of the fluid flow.

Candace subconsciously knows about Perry's missions since "The Ballard of Badbeard"
She kept herself in denial about it, hoping that any technicolor moss make her go on high.
 * 'The Wizard of Odd' proves this theory further, as

Candace will have a nervous breakdown eventually and go completely insane, killing Phineas and Ferb
The Candace from the future? She's a clone made by Carl but he couldn't make Phineas and Ferb because he ran out of important chips. Linda was hiding the truth from the boys when they traveled into the future.
 * Jossed. Two words; "Aunt Isabella".
 * She only kills one of them, then. Or Isabella was integrated into the family before her spouse was killed.
 * This WMG has "sketch on Robot Chicken" written all over it.

Candace is actually Kevin Destructicon
I'm not sure how (maybe she's a Transformer, given that last name there), but it explains why the Zebra calls 'her' Kevin.

Candace is secretly a cannibal
It would explain the third eye in her stomach in the episode "Journey to The center of Candace".
 * Or just an animal with a white eyeball.

Candace is jealous of the bond between her brothers.
Before Ferb came along, Phineas and Candace had only each other. And Candace doesn't have a sister or anything (just Stacy)--she probably wants to be closer to her brothers. Because she's terribly insecure, she doesn't realize that they love her to death and really want her to build stuff with them. It's kind of like how she spent years convinced that she had to impress Jeremy, even though it was obvious that he was crushing on her just as hard as she was on him.

Candace is the reincarnation of some guy named Kevin
Simply because there has to be some reason why the talking zebra always calls her Kevin.
 * To take that even further, the zebra was Kevin's pet.
 * To take that even further, Kevin was some kind of warrior and the zebra was his mighty steed!
 * To take that even further, the zebra is not only Kevin's mighty steed, but a magic zebra, and that's why he can talk. Also, Candace and the zebra will go on all sorts of epic adventures together.
 * And maybe Perry is the reincarnation of the Talking Zebra.

At some point in her future Candace will become a police officer.
And she'll be able to get over her obsession with trying to bust her brothers by busting wanted criminals instead.

Candace's obsession with busting/animosity toward the boys is beginning to fade
As of the last two episodes (the Candace Disconnected and Bad Hair Day parts, at least) she's actually asked the boys to help her out (with Stacy's prompting, in the latter episode), which is something she definitely wouldn't have done in the first season. This may be leading up to the Lost Aesop of Wizard of Odd, which encouraged Candace to stop being so single minded, and, instead, have fun.
 * I think her busting/non-busting has remained pretty much constant. I haven't made a graph or anything, that's just my perception. There have been non-busting episodes since almost the beginning. About a quarter of the way into Season 1 there was a string of three non-busting episodes in a row. So it's not like this is a new trend.
 * This troper agrees with this theory as I once went through a list of the episodes and noticed that there were a surprising number of episodes in which Candace either says she's going to bust her brothers but doesn't follow through on it, or where she doesn't even bring up her busting obsession at all.
 * The busting to non-busting ratio has shifted somewhat. Of the 24 episodes aired so far in Season 3, there are 12 where Candace either doesn't try to bust Phineas and Ferb, or makes only a brief attempt. This is a substantial drop from Season 2, where she made a considerable effort to bust them in 42 out of 63 episodes (not counting dreams or clip shows). Surprisingly, she was less busting-crazy in Season 1, but still made serious attempts in the majority of the episodes (27 out of 45, not counting "The Monster of Phineas 'n' Ferbenstein" or "Phineas and Ferb Get Busted"). In conclusion: This troper has too much spare time.
 * OP here-- it just occurred to me that the above might not have been the best explanation. I meant that Candace is getting a lot more lax when it comes to asking the boys for help, and may end up making the equivalent of a Heel Face Turn sometime after Across the Second Dimension, and end up teaming up with them more often than not. (Whether that totally eliminates the busting thing or not is subject for debate, though.)

Candace not only wants to bust her brothers, but to prevent Isabella and Phineas from ever going out.
She definitely knows about Isabella's crush on Phineas (vide "Phineas' Birthday Clip-o-Rama"). If she busts her brothers, Isabella would never be allowed to come over to their place again.
 * Candace and Isabella seem to be friends, or at least they get along well together (vide "Got Game," "Out to Launch"). She's never tried to bust Isabella for her contributions to Phineas and Ferb's projects (she was the only kid involved in the charade in "Not Phineas and Ferb" whose mother Candace didn't call), and I don't think she's ever so much as scolded her. So I really don't think Candace is trying to drive a wedge between her and Phineas.
 * Remember that the cruelest comment Candace has ever made to Isabella is: "Or Ferb".
 * If "Or Ferb" occurs it will be because Isabella is trying to make Phineas jealous, but this backfires because he's gay.
 * Ferb isn't gay. Please refer to the page image for Getting Crap Past the Radar here. Thank you, and have a nice day.
 * I think he's saying Phineas is gay (which I also don't agree with).

Candace can see into other dimensions...

 * ...but only when her sense of reality is 'weakened'; whether it be because she thinks she's hallucinating or dreaming. In Across the 2nd Dimension they run through a dimension with the Talking Zebra and he's holding a newspaper that declares that Kevin(what he calls Candace) won the Presidential election, and in an earlier episode the zebra even mentions voting for Kevin/Candace.

Candace is Anne Frank's second life.
Candace might actually be Anne Frank's second life... She's got the enlogated version of Anne's hairstyle...plus this troper bets the show focuses on her so much is because the whole thing is actually written all on a diary she keeps....hence the show is technically about Candace, and the title is what she titled her diary, keeping record of their adventures and inventions...so phineas and Ferb are Nazis? Jossed: Phineas and Ferb are so not Nazis, but the Candace/Anne theory kinda makes sense... plus, the Smile Away Reformatory Candace/Perry dreamt about was technically a child-friendly parody of Auschwitz. and ATSD gets A LOT from World War 2, with the dictators, hiding, and resistance...

Is Phineas and Ferb somehow set in a city which suffered World War 2? (makes sense because of "Battle of Danville")

Candace was Isabella's mentor when Isabella was a Little spark.
Candace being Isabella's mentor would explain why they have such a good friendship.
 * "Fireside Girl Jamboree" had implied that Candace hasn't been a Fireside Girl before, so Jossed?

Candace is still a vampire.
Since Word of God has established a firm no magic rule, it must be a disease, like in that one book. The bat gave the virus to her, and all the things that could have tricked her into thinking she was a vampire being around was just another one of those cruel coincidences that run her life. She really did get turned into ash by the sun. Phineas and Ferb resurrected her and she is now immune to the burning light of the Daystar (it's like chickenpox, you can only get it once) so the boys thought she was cured. She now lives a secret double life undercover of darkness.

Candace will have her busting urges repressed in some way.
20 years into the future, Candace is much more relaxed and normal, but the moment she sees 'young Phineas and Ferb' again, she slips into her old habits. Considering Candace as she currently is, she'd go insane if she tried to do this by herself. So sometime into the future, when the boys have grown up and left for their separate lives, she'll be hit by an invention that does this (either from the boys or Doofenshmirtz), allowing her a good amount of time to mature mentally before the younger boys come later.

Candace, and Ferb to a lesser extent, have some knowledge of Perry's role as Agent P
In Ready for the Bettys where both the brothers come upon Doof's secret lair, Phineas immediately thinks that they're Ferb's, though, throughout the episode, Ferb is never allowed to speak fully about how the inventions there were not his - but Doof's. Also, Candace (in her subconscious at least) must realize at the back of her mind how strange her encounters with Perry have been - changing bodies with him, sharing his dream in Phineas and Ferb Get Busted, and tripping over him in his Agent hat in Come Home, Perry, though since her life is as unusual as it is already, Candace probably dismisses these "Agent P" apparitions as dreams or illusions.
 * She wouldn't remember Perry's experience in her body, but her own experience in Perry's.
 * Also, she didn't share her dream with Perry, it was a Dream Within a Dream.
 * In "Phineas' Birthday Clip-o-Rama" Ferb seems to know enough about it to be able to parody Perry's entrances to the lair.
 * In Phineas and Ferb The Movie: Across the 2nd Dimension,.

Perry's a girl
His (her? its?) theme song says he's an 'egg-laying mammal of action'. Guys don't lay eggs.
 * That same song explicitly states that 'he' is a he. The egg-laying part can refer to the species itself. What's more, when watching soap opera in one episode, he/she clearly appears to be unshaven (if it's possible for a furry animal). So even if she's a female, she has some serious hormonal problems.
 * Also, male platypi have a poisonous spur in their hind foot. Doofenshmirtz should have been paralyzed by now because he's constantly kicked by Perry.
 * Pet platypi probably have their spurs removed (much like pet skunks have their stink glands taken out)
 * Plus, Perry's a gender-neutral name.....and Candace was sweating milk in his body that one episode....
 * This would make her a lesbian according to her theme song or at least the women in her theme song's video.
 * Or they just can't tell by sight.
 * I've always assumed that when Candace and Perry switched bodies they actually swapped some of their gender characteristics as well. For example, because Candace is a girl and is now in Perry's body Perry('s body) is now able to sweat milk. Likewise, Candace('s body) does not have breasts (however we can't tell because she was already flat-chested), and everything was switched back to normal when P&F got Candace's and Perry's brains in their correct places. As for the leg spurs, they were probably removed when Perry became the family pet, or perhaps already had been when he was adopted. In short, Perry is undoubtedly male.
 * Platypode are recognizable by their ability to lay eggs, as well as being semi-aquatic; therefore, they're not necessarily saying Perry lays eggs, they could just be saying he's a platypus without using the actual word.
 * (Nice use of "platypode") Also, considering he is a pet, isn't it likely that he would have had the spurs removed when the family bought him, like how skunks have their stink-glands removed?
 * Yes, but I can't see why the family wouldn't spare themselves the expense of the operation if the deviation from the ideal was one as small as gender.
 * Perhaps domesticated platypi don't have spurs.
 * Or the OWCA removed the spurs so that Perry wouldn't kill or permanently harm Dr. Doofenshmirtz.
 * Plus it's been established that Doofenshmirtz Wouldn't Hit a Girl. Granted he might not be able to tell either.
 * Yeah but neither the boys nor Candace seemed surprised that Perry had laid an egg (well, he actually didn't but they thought he had)
 * Obviously the blame then falls on the lackluster Danvillian school system and their subpar sex education program.
 * Also noticed that when Perry sweats, milk comes out. Even the times that Candace wasn't in its body.
 * It's pretty obvious in cartoons that sweat could be white.
 * That's not biologically possible.
 * If you mean the part about sweating milk, yes it is. Platypi sweat milk.
 * Nope, in "No More Bunny Business", Perry sweated normal sweat, which would be blue. As for sweat being white, animation error/lighting?
 * Maybe Perry's a Hermaphrodite.
 * In the episode "Perry Lays An Egg", when the new transporter device sucks off the hair of Perry, you can clearly see he's wearing boxers.Pause at 1:02 Don't even give me the "girls wear boxers", because there's no bra.
 * That's because he's a platypus, duh. They don't have mammary glands.
 * Ahem. He's a semi-aquatic egg-laying MAMMAL of action.
 * Sorry, my mistake. They do have mammary glands. My point still stands, however, as they have no nipples (they sweat out the milk). There's literally nothing to cover.
 * Well, except for the bottom of course.
 * In here, there's a Perry model with boxers. It has an cross on it? Do breastless females have a cross on their chest when their naked?
 * In "My Fair Goalie", Perry goes to the boy's bathroom instead of the girl's bathroom.
 * Which raises a question as to why Doof has a boys' and a girls' bathroom in his apartment...
 * Because his lair is public? Because the creators want to reveal Perry's gender?
 * He has a teenage daughter, do you really think Vanessa would agree to share a restroom with men?

The Flynn/Fletcher kids either think or know Perry's a girl
Which explains why they treat Perry as a girl in some occasions. (Though, Candace doesn't know that female platypi "sweat milk"), they called Perry a boy because they're used to calling pets dudes.

The Flynn-Fletcher family knows that Perry's a boy, they're just aware that Perry might be a girl.
Candace doesn't mind dressing Perry in pink, and Phineas and Ferb dressed Perry in coconut bras. Yet they still call Perry a he. They know he's a boy, but they might have think that there's a possibility that he's a girl, so they would treat him like a girl occasionally so they wouldn't be surprised.
 * A male platypus has internal testes, and the penis is retracted except when it's aroused, so it's believable that the family would be unsure of Perry's gender (if we go with the common guess that his spurs were removed, and assume that they were already gone by the time they got Perry second-hand). It is possible to sex a platypus by sight, but it takes some research to know what you're looking for.

Perry is Candace's twin brother turned into a platypus.
Which explains why Perry is mind-linked with Candace and is very close to her. As for why Perry's old while Candace's young, it's because platypi age faster.

Perry the Platypus is Candace... somehow. (Yeah, I know it's rather Squicky)
Some evidence:
 * They're both not treated seriously (and probably treated quite poorly). Candace isn't treated seriously when she tries to bust her brothers, while in Perry's case, Major Monogram and Carl would occasionally joke around with him despite the serious personality he has.
 * In "Gaming the System", Perry wore a ball gown which is suitable for Candace when struck with the Ball Gown-inator. When Phineas, Ferb, Isabella, and Dr. Doofenshmirtz were struck, the dresses fit.
 * The song Perry the Teenage Girl is supposed to describe Perry the Platypus in Candace's body (justified as one of the lyrics says that she's a girl with a platypus's brain). But then the song is more suited with Candace, complete with a parachute with Jeremy's face on it.
 * Your Mileage May Vary, but Alt. Candace acts quite like Perry the Platypus. Just look at the preview!
 * In the Across the Second Dimension trailer, the mystery figure appeared right before Perry the Platypus when the audience were supposed to think that the mystery figure's gonna be "the agent that fits the bill". Then in the movie itself, the mystery figure appeared and the audience were supposed to think that the mystery figure is supposed to the leader of the Resistance, which turns out to be Alternate Candace.
 * In the movie, Alternate Candace also acts similar to Perry. They both deeply care about Phineas and Ferb, they're both Badass, serious, and doesn't have time to do their normal daily things.
 * Perry the Platypus actually knew The Talking Zebra, according to "Phineas and Ferb Get Busted!"

Perry uses his secret agent skills to the sneak into Candace's room
Candace keeps waking up to find Perry on her bed. She keeps complaining about Phineas and Ferb letting Perry go in her room, which suggests she keeps her door and window shut at night and she can't see any other way he could get in. I think he keeps sneaking in using his secret agent skills just so he can sleep on her bed. It's quite cute really.
 * So he goes to great lengths to repeatedly enter the bedroom of an underage girl without anyone's permission so he can spend time in bed with her? I think Fridge Brilliance, Fridge Horror, and Getting Crap Past the Radar have just met.
 * Especially if you believe one of the WMG's above, that Candace and Perry are telepathically linked somehow.

The secret weakness of Perry the Platypus is...

 * The echidna, an insectivorous egg-laying mammal of evil. It's got more than just mad skills (it's got non-detachable quills).
 * If it's a super-strong, red echidna with dreadlocks, then this troper will find a way to watch the episode where it appears.
 * Echidnas hate Platypi or... Platypuses um... Platypeople err... whatever, anyway Echidnas hate them because they get all of the credit for being egg laying mammals.
 * Non-detachable quills? The creators seriously need to throw this in somewhere. Seriously.
 * The Enemy of the Platypus is Man.
 * Semi-jossed, since Perry the Platypus (and therefore Agent P) is a domesticated platypus.
 * Semi-confirmed, since "Doctor" Heinz Doofenshmirtz is the archenemy of Perry the Platypus.
 * Really good cheese.
 * Perry the Platypus' weakness is anything endangering Phineus and Ferb.

Phineas and Ferb are start to getting tired of Perry
In the episode "Lizard Whisperer", Phineas and Ferb sound more concerned about where their new pet is than where Perry is. This could happen because they are starting to get tired of Perry's disappearances...they are trying to find a new pet.
 * Considering Perry always comes back just fine, they probably just don't see a reason to be concerned anymore.
 * They still allow Perry to ride on Steve with them, though.

Perry the Platypus' weakness is not wearing his hat
Because he reverts to an animal when he does. I could be wrong though.
 * Yeah, you were. Sometimes Agent P wears disguises while not wearing his hat.
 * True, but those disguises could have been specially designed.
 * How about at the episode "It's All About Time"? Perry wasn't wearing a hat during Mr. Feelbetter's show, isn't he?
 * Curse you TV tropes writer!

Irving knows Perry's Secret
I mean this is Irving after all. If he follows Phineas and Ferb every seconded of every day then he MUST know by now
 * that's the exact reason why he wouldn't know. If he follows Phineas and Ferb every second, hes not concerned with Perry and is focusing on them.(especially to hear them say the line "wheres Perry?") Although he probably has records of times Perry disappears and comes back...
 * In "Phineas' Birthday Clip-o-Rama", he has clips of pretty much everything...incluidng a scene with poofenplotz. So he knows Pinky's secret, so he must know perry's!
 * Due to all the fourth wall bashing in that episode, it probably can't be treated as canon. Not to mention that he also has clips from episodes that turned out to be all a dream.
 * Maybe he's that creepy of a fan.

The entire thing is Perry's dream.
The entire series ends with the episode "Phineas and Ferb Get Busted." Perry is a normal platypus. Phineas and Ferb and Candace are all completely normal in real life. Perry just had a wild and extremely long nightmare, perhaps even a recurring one.
 * But the show is still going. It seems too good to end soon - and Disney Channel can stretch out shows for a long time.
 * The show is in non-chronological order, a la Suzumiya Haruhi.
 * Though some episodes do make reference to past episodes, notably in Doofs "I already told you that flashback" moments to Perry, and sometimes the boys/friends/Candace reffer to past projects... And of course the episode dealing with the Camera across the street that has recorded everything from the projects to Perrys comings and goings.
 * So? It could still be in non-chronological order.
 * As long as they don't make a Groundhog Day loop à la Haruhi, endless eight anyone?

We see Danville (and the rest of the world) through Perry's eyes
Hey, it explains why everyone's so cartoony looking.
 * How so? Real platypi would see everyone real, but from a low vantage point.
 * This was supposed to be like the "We see Springfield throught the eyes of Maggie Simpson" theory.

Perry used to be the Flynns' Battle Butler/Babysitter before Linda met Lawrence.
When Perry was a human, he was hired as a babysitter by Linda right after she married her first husband. Why a man? So the babysitter would "replace" Linda's husband who is always busy doing his complicated inventions. He played with Candace so much they had a special bond with each other. So does Phineas (but not as much as Candace, who played with Perry longer than Phineas). After Linda's 1st husband died or disappeared or something like that, Perry started to stay at home longer. Somehow, after Linda and Lawrence first met, Perry turned into a platypus agent.
 * Perry might be Phineas and Candace's biological father, having memories of Phineas and Candace and all. But the problem is that he doesn't seem to have some special relationship to Linda Flynn. Thus this theory showed up.

The "Perry and Candace are mind-linked" theory is true, and Perry tries to clean up after P&F on purpose.
Perry knows that if he doesn't ruin Doofenshmirtz's life so that he can steal his inventions to do this, then the "Phineas And Ferb Get Busted" episode will actually happen.

Perry isn't an ordinary platypus.
Perry isn't a true platypus. He may be a robot or he used to be a human.
 * First of all, Perry wouldn't eat things that were part of his diet, as shown in "Brain Drain" where Perry was in disgust when he ate a bug. Platypi are insectivores, you know.
 * On the other hand, Perry usually eats platypus food, which (according to Linda in "Does This Duckbill...?") is made from grubs and earthworms.
 * It might be because platypi usually eat insect larvae, not the adults (which they aren't very good at catching).
 * Another explanation just occurred to me: It might have disgusted him because it was filthy, not because it was a bug. It was a fly from a landfill, after all.
 * Platypi don't dream. No REM = No dream. Perry did dream during "Phineas and Ferb Get Busted!".
 * That's a misconception. See: http://www.nature.com/news/1998/980813/full/news980813-5.html and http://www.npr.org/templates/story/story.php?storyId=90228565
 * That doesn't stop the fact that (most people think that) platypi don't dream.
 * Perry uses a small breathing device (similar to the one James Bond used in Thunderball) and sometimes full scuba gear when swimming underwater, even though platypi are capable of functioning underwater for reasonably long periods.
 * There's one scene in "Interview with a Platypus" where Perry doesn't need a breathing device while swimming.
 * Perhaps he could only swim for a while, unable to swim as long as an ordinary platypus.
 * Obviously, he acts like a human, besides acting like a mindless pet in front of the Flynn-Fletcher family.
 * He can't be a robot, since in "Oh, There You Are, Perry", Perry is seen as a younger platypus.
 * A younger robot platypus?
 * In the background of Perry's underground lair (more like Major Monogram's) there are some equipment that were unused, like a jetpack. One of the suits looks like it's human-sized, which means there might be some human agents working for the O.W.C.A.
 * In "Quantum Boogaloo", it is shown that Perry would live a longer life-span than even the domestic platypi in real life.
 * He also could be a platypus with a microchip in his brain that enables him to do almost everything.

Perry is a genetically modified platypus.
Thus explaining his intelligence and ability to dream.
 * My bet is there's something about those hats. Given that any critter (even Kevin herself) turns secret agent with a hat.
 * Then how could the "mindless animals" even have the intelligence to wear the hat?
 * Perry still acts like a human if he doesn't wear a hat (see "Oil on Candace", "It's About Time!"). Or perhaps Perry's just an exception, which is only possible further proof that Perry used to be a human.
 * And he's green because of the genetic modifications! But wait... ALL the platypi in Danville are green.  So they're all mutants!

If Perry the Platypus will have some time to speak in human language, he'll be a Badass Baritone.
Well, Meap got that "superhero voice" when he speaks, so why not Perry and his badass voice. Maybe with Tim Curry as the voice actor, but more "official", since during the episode where Perry "talked", "A Hard Day's Knight", he didn't really talk. It's only the professor he disguised as recording.
 * FWIW I've always imagined him sounding like Samuel L Jackson.
 * Daniel Craig or Sean Connery, just 'cause.
 * This troper, thinking that Perry's Phineas and Candace's biological father and all, thinks that either Dee Bradley Baker or Danny Jacob should do Perry's human voice.
 * Maybe Meap's Badass Mustache translator will work on Perry.

Perry is an omnipotent being.
Perry apparently said "I may swim with my eyes and ears shut, but hear and see all. Beware evil doers!" in the Agent Communicator activity in the Platypus Day page. Also, he seems to know how to do everything and knows everything (as seen in "Phineas and Ferb Get Busted!").
 * He even had time to see Phineas and Ferb's projects, proved in Across the Second Dimension.
 * Perhaps he's the "Mysterious Force".

Perry is not awesome
He's a level 10 and Doof is level 6 for example. He is just better then doof. My proof is "THE REGURGITATIOR"
 * Maybe, but out of all the OWCA's secret agents, he's definitely the awesome...est. Proof? Just watch the movie.
 * Perry's not awesome? *universe implodes*
 * He did promptly defeat the Regurgitator after being set free though. So there are several alternatives to this:
 * 1) Perry is awesome at fighting but not (as) much at getting out of traps. Which isn't that far fetched; after all, there've been a few times where he got out of Doof's traps through sheer luck or outside help.
 * 2) Perry was biding his time and would've gotten out in the end.
 * 3) Perry was Genre Savvy and knew he would get out somehow, even if he didn't have a specific plan in mind.

In "Perry the Actorpus", Perry will talk because of what Phineas and Ferb did.
Well, he did became "spokesanimal" for Totally Tools according to the summary, and they haven't revealed Phineas and Ferb's Big Idea from the episode, so why not they make Perry win the competition by making him act like a real spokesperson. Darn, I must be too obsessed with him talking!
 * Jossed.

In "Perry the Actorpus", Perry will just make a growl as a "spokesanimal".
The boring alternative.
 * Jossed. It's even less interesting as Perry only served as a model for the product.

Perry is the stillborn/miscarried child of Linda and Lawrence, reincarnated as a platypus.
Shortly after Lawrence and Linda got married, they conceived a child together, but he didn't survive. Candace, Phineas and Ferb knew about it (though Phineas and Ferb's knowledge was less clear, since they were little kids), and, to lighten the mood a little after such a sad event, their parents let them pick out a pet.

Their younger brother was going to be named "Bartholomew", so Phineas and Ferb chose that name, innocently seeing their new pet as a Replacement Goldfish. Their parents made them change it, volunteering the name "Perry" instead--would you let your kids name a new pet after a recently-deceased sibling? Little did they know that, by some miraculous twist of fate, their child had been given a second chance at life.

Not much more to say here, except that if you watch The Movie, Perry acts very brotherly toward Phineas...
 * Or fatherly, which is part of the theory that he's Phineas and Candace's biological father.

Perry was adopted after Bucky's dea--
-ahem- After Bucky went to go live on kindly Old Man Simmons' farm. Lawrence and Linda elected to adopt a new pet instead of suffering constant questioning about Bucky, and let the kids choose in an effort to distract them. (Which is odd, because platypodes don't do that much.) This is a common way people get over the loss of a pet, so it's not too far out there...

Phineas and Ferb's pet platypus, Perry, is actually Agent P!!
Just think about it, man! Whenever he disappears, we cut to agent P getting his mission, and he doesn't appear again until Doof is thwarted! Also, his name begins with P!! Coincidence??!! I! Think! NOT!
 * My god... This makes so much sense...
 * I doubt it. After all, he is just a platypus. They don't do much.

Perry started off fighting Doof for pleasure.
Honestly, the nature of their fighting seems different in "Neme-Nemesis," with Perry smiling, unlike in the episodes where he mainly seems irritated by Doof's shenanigans. Also, the fact that someone as epic as Perry was assigned to the lowest villain on the charts seems a bit strange. Therefore, I think Perry started out as a sort of weird stalker who foiled Doof's plans until he learned that there was a job in it.

Perry "sees" Phineas and Ferb's inventions with Danny Jacob's voice.
Related to the theory that Perry's Phineas and Candace's biological father and that Phineas and Candace's biological father is voiced by Danny Jacob. Perry "sees" the boys' inventions via soundwaves while scanning them for some data. Perry doesn't "see" Doofenshmirtz's inventions, so Danny Jacob's voice is absent.

Alternatively, Perry sees Phineas and Ferb's inventions with some spy camera, and each time Perry spies on them, Danny Jacob's voice occurs, which could come from any living being, making it some kind of "leitmotif". This explains why kids, women, and even aliens are voiced by him.

If Perry turns into a human, he would look like the Guards in "Wizard of Odd".
Long nose, neck possible as wide as the head, wide eyes, yep, similar to Perry.

Lindana's singing rival is Madonna
just think about it isn't lindana simular to Madonna? and if she guest stars they will have a epic sing off

Linda and Lawrence are each others Old Flame reunited
It is a fact of the show that Phineas and Ferb are in fact step-brothers, and in "Phineas and Ferb get Busted" Candance remembers meeting Ferb when him and Phineas were toddlers; however in "Dude, we're getting the band back together" shows Lawrence and Linda sharing their first kiss in a Love Händel concert somewhere in the late 80s. How is that possible? Simple, Linda and Lawrence were together back then, but broke up and went to marry other people (one presumes), years later, single again with kids this time, they met again and got married one another.
 * Jossed. According to Word of God, Love Händel was a rare 90s Hair Band.
 * But there are serious problems with that explanation. In the flashback, the audience is shown with stereotypical 80s hair and clothing. Plus the TV special said "but styles were changing..." That makes it clear that Love Händel's musical style had widespread popularity at the time, meaning it must have been the 80s. They were probably one of the many New Wave groups that disbanded between 1986 and 89 (e.g. a Flock of Seagulls, Spandau Ballet, OMD, Culture Club, Journey, A-ha, Kajagoogoo, Men at Work).
 * Also, nothing in "Phineas and Ferb get Busted" can be taken at face value since it was a dream within a dream.

The parents know everything their kids get up to, they just don't care.
Presumably they had equally crazy fun childhoods, and as a result aren't too worried about Candace apparently being dangerously delusional.
 * This is very possibly canon with the father since he knows about the monster truck rally in "It's a Mud, Mud, Mud World". Both sets of grandparents certainly know as they've been party to several of the boys missions.
 * The father knowing? Established canon. The mother knowing? Not so canon, but an interesting possibility. She uses obfuscating ignorance to keep Candace in line. After all, if her teen daughter is wasting her energy obsessing over her brothers, she isn't out experimenting with drugs, alcohol and/or boys.
 * The "mom's birthday" episode probably establishes she knows. After all they did surely she has to suspect something.
 * Nothing happens in the episode that displays the boys' amazing inventing/building skills. Remember, the first thing she saw was the breakfast buffet, where adults were already present. She probably assumes the adults built the stage and setup the satellite link to Lawrence. It's mind-bogglingly expensive (could confirm Danville as a haven for eccentric billionaires), but she saw nothing to indicate that Phineas and Ferb did everything.
 * Besides, when she did find out about Phineas and Ferb's inventing skills, her reaction wasn't that good or happy. See episodes 'Phineas and Ferb Get Busted' or 'Phineas and Ferb's Quantum Boogaloo' to get a better understanding of my gist. Then again....

Lindana will make a serious (albeit temporary) comeback sometime during the run of the series.
The fad, not the person. I'm guessing Americans Love Portugese Lindana Discography.
 * Well, there will be a Season 3 episode where Phineas and Ferb knows that their mother is Lindana...
 * It just aired in Latin America.
 * Confirmed.

Linda adores geeky people.
Which is why she dated Heinz Doofenshmirtz, Lawrence Fletcher (whom she will marry later on), and possibly Phineas and Candace's biological father who might have almost the same talents as Phineas.
 * And Baljeet

Without somebody pointing it out, Linda sees the "Big Ideas" as reasonable projects
It's a bizarre mix of Weirdness Censor, Invisible to Normals and-- quite possibly-- Elephant in the Living Room. If she did happen to see the inventions on her own-- without Candace pointing them out, etc.-- she wouldn't see anything wrong with them. For instance, if she'd seen the real fort from Thaddeus and Thor, it would have looked like the 'homage to old-school construction' that she saw in-episode; if someone does point out how elaborate the projects are, though, she's able to see them for what they really are -- which explains the reaction in Phineas and Ferb Get Busted (which, admittedly, is non-canon) and She's the Mayor, where she did get mad. It doesn't apply to the projects like the ones from Dude, We're Getting the Band Back Together and Ladies and Gentlemen, Meet Max Modem, where it isn't readily apparent that the boys are responsible.

Lawrence was once abducted by aliens.
There's already the alien trope above, but Lawrence is obviously human while Ferb's grandfather really looks like him, so maybe Lawrence was once abducted by aliens à la The Sims and had Ferb.

The new episode, "Bullseye!", will include Evil!Lawrence and Linda (Kikikikiki).
Just look at the plot and tell me this isn't true. Given the show's history, it's possible that the clip show episode wasn't aired chronologically, which only adds to this WMG's liklihood.
 * Semi-confirmed. Lawrence did do the "Kikikikiki"-like laughter once he got evil, but not with Linda.

Lawrence and Linda are a widow and a widower respectively.
After their spouses died they each got a sex change and when Linda was Lindana she was crossdressing and she was pretending to be a girl when she dated Doofenshmirtz.
 * What?

Isabella isn't just half jewish. She's half Israeli
This WMG requires a bit of understanding of Israeli circumstances to comprehend. Since Israel is constantly surrounded by/attacked by people obsessed with their destruction, IIRC, all adults in that country are required to serve some amount of time in the armed forces (typically in the reserves) so that should war be declared on them, everyone will be able to defend themselves. How does this relate to Isabella. During the lyrics to the end song of the Family Christmas Special, she mentions her family coming from two cities, one of them being Jerusalem. We also find out that her father is very much alive, which josses a WMG about a dead fireman. This WMG suggests a possible origin to her Action Girl tendencies and some of her skills. That would possibly be due to her father teaching her some of the things he may have learned in his time in the armed forces, assuming that he is indeed from Jerusalem (making him Israeli). This WMG also suggests that anyone who tries to harm Phineas may very well be in some serious trouble as there is a very real possibility that she might know Krav Maga, which is a martial arts style invented by the Israeli military that has a reputation for being one of the most brutal, no-nonsense fighting styles around.

Isabella has a centaur fetish.
In "Isabella and the Temple of Sap" when she's in 'Phineas-land', he randomly transforms in a centaur. Make of that of what you will.
 * Well, did she ride this Phine-taur? Many girls ride horses because riding them can induce orgasms (Just stating facts!) so in her mind this would be G-rated sex.
 * Yes. Yes she did. Twice, in the same episode, to boot.
 * Isabella loves horses (shown many times) and loves Phineas (ditto), so it's not surprising that the two would come together in her mind...

Neither Phineas or Ferb will marry Isabella.
In quantum boogaloo, Amanda said that Isabella looks like Aunt Isabella, which means that Phineas or Ferb could have just married any Isabella look-a-like who happens to be called Isabella.

Aunt Isabella is married to nobody.
If Izzy married one of the brothers then she'd have to give the other one up, and P&F share everything. No doubt she has kids from both brothers.
 * Three problems with this theory. 1. Phineas and Ferb will probably not share things as much as they do now once they get older. 2. Isabella just wants Phineas and doesn't want to divide her attention between him and Ferb, as shown in "Out to Launch" and "Canderemy." 3. It's not what either camp of shippers want to happen.

Isabella is full of pent-up rage
...And it will all burst forth someday, to the shock of everyone. Actually, this might already be canon. I haven't seen every episode yet.
 * Calling this as her Crowning Moment of Awesome in Across the Second Dimension.

There is still some Doofelium in Danville's atmosphere, and Phineas and Isabella are especially susceptible to it.
A canonical explanation for why their voices got higher as the series continued.

Isabella's crush on Phineas stems from the day they first met.
Either it was love at first sight for Isabella or Phineas did or made something special for Isabella that made her fall in love with him.

Isabella will lead the O.W.C.A. or at least Major Monogram's division.
At Doof Dynasty, where Major Monogram was a regent while Isabella's a princess. It suggests that Isabella has a close connection with Major Monogram and she'll replace him after Major Monogram's too old.

Isabella joined the Fireside Girls to try and impress Phineas.
Or at least get his attention. It wouldn't be that surprising, considering her strong feelings for him.

Heinz Doofenshmirtz is the true favorite son
Think about it. Every last value his parents have and their approach to anything and everything was/is freakish. They wanted to produce a mad scientist son, especially since their ancestry includes at least one. Roger is the disappointment, whom they baby with attention and praise because he could never handle being truly E-VIL!!!!

At some point,Doofenshmirtz will become a Woobie, Destroyer of Worlds.
Let's face it, he's had a miserable childhood and is the biggest Butt Monkey (even more than Candace) on the show.At some point,he'll snap.Since this is a kid's show,he'll be the G-rated version of this trope, and attempt to destroy the whole tri-state area.Making himself a bigger threat,Perry will organize a group to calm him down,and he'll get his happy ending.The reason for the dismal alternate future in Quantum Boogaloo is because something happened to Vanessa, causing him to go through the despair and Moral Event Horizon

Heinz Doofenshmirtz doesn't think that his family is his real one.
Due do being abusive, Heinz believes that he came from another family, and compares his current family to beasts. Thus thinking that he was raised by animals. Pretty harsh, isn't it?
 * This troper actually had a similar theory in which Heinz's parents were actually his step parents who ended up adopting him. This is due to him mentioning that neither of his parents showed up on the day of his birth (which is just odd) and that he was raised by ocelots. It might also explain why his parents favor his younger brother Roger over him.
 * Actually, in this theory Heinz's abusive parents were his birth parents. It's pretty much proven by their similar looks. And this troper also thinks that it's their country tradition to be harsh to their first child so they could become mentally and physically strong and take care of the family when the father dies, which Heinz failed to be because he is some neurological disorder savant. So Heinz compares them to ocelots.
 * Well, I never meant that my theory was completely similar to yours, I just meant they were kind of similar. Also, in my theory I would probably explain his abusive parents similar appearances to him by saying that they were distant relatives of his. Although your theory makes sense too.
 * Jossed in the ocelots part. In Across the Second Dimension, he said that he was literally raised by them.

Doof will reverse engineer the Super Computer.
Plus, during a flashback of Doof explaining to Perry how he built his Know-Everything-INATOR, "What Do It Do" will be heard in the background.

Doofenshmirtz shall one day realize that he was successful in something his brother Roger didn't get.
And that is Vanessa. The loser actually managed to have a daughter. This could count as a D'aww moment.

Doofenshmirtz will make a Procrast...
Inator. Can't you hear it? "See, Perry the platypus, it's a pun on the word procrastinator! Isn't that funny?"

Doofenshmirtz intentionally caused the Regurgitator's defeat
In a rare bout of genre savviness, Doofenshmirtz used several of the mistakes that regularly lead to his own downfall to his advantage in order to sabotage the Regurgitator.

Doofenshmirtz was high during the whole Lake nose incident.
Forgetting why he wanted to drain the lake of its zinc? Jumping his train of thought between tracks the way he did? The hot wings, and his Seinfeldian Monologue with the kicking back on the floor and rambling on? All a Parental Bonus for him being on something.
 * I'm sorry, I just couldn't stop laughing after reading that; it also explains why he tried to futilely stuff himself down the toilet...
 * you know thinking about it.... that kinda reminds me of my boyfriends birthday party last year
 * He was high because of the pressurized air he was breathing!

The entire series is Doofenshmirtz's Dream
In real life, Doofenshmirtz is a widowed, drug addicted, failing inventor. Phineas and Ferb represent his sadness over his lost childhood. All of Dream!Doofenshmirtz's inventions are what Real!Doofenshmirtz wishes he could build, and the evil he does represents his drug addiction. Perry represents his real life vitriolic best bud who struggles to help him with his addiction, and the OWCA are the rehab that Real!Doofenshmirtz's drug-addled mind believes Perry is working for. His ex-wife and daughter represent... his ex-wife and daughter.
 * So, in real life, his wife don't believe there's no such thing as drug addicts?
 * No... Just... no. This is just too f***ing depressing!

Heinz Doofenshmirtz is suffering from Savant syndrome.
His disability caused his parents' neglect towards him, and he has the talent in building machines and hand cooperation.

Doofenshmirtz has something going on with his back up dancing girls
Think about it, they always seem to be on hand waiting for random musical numbers... cheerful and blonde... heck they even work on CHRISTMAS EVE... It has to be more than his Wifes Alimony that gets them to show up so regularly.

Doof is actually adopted.
Well think about it. His actual biological parents had him as an accident and decided to drop him off with his parents. But he knows nothing about it. His adoptive parents couldn't really just leave him, but they didn't exactly care too much for him either. It you really think about every single thing we know about his childhood, they would all come together in some sort of Fridge Brilliance
 * I've already made a similar WMG along time ago. See the above entries for more details.

Doofenshmirtz was once a Broadway composer under an assumed name
Part of the divorce settlement was that Charlene got the royalties, with some of that money going to Heinz as alimony payments. It would explain his preference for, and skill with, all those show tunes he puts on for Perry. (Lampshaded at one point when Perry picks up a program sitting on the floor next to the chair he's bound to.)
 * In the commentary for 50 feet sister, Doof admits to Monogram to own a platinum disk.
 * He also might be the one who choreographed the musical scene in "Bullseye!". You know, being in the center and stuff.

Doofenshmirtz will eventually build a lampshade-inator
The creators do go on tvtropes, and have included trope jokes in the show in the past, so it figures this will eventually happen. It will be a device to hang a lampshade on everyone and everything IN THE ENTIRE TRI-STATE AREA! Seriously, though, if anyone from the show ever reads this, it would be really awesome to do this.

Doofenshmirtz is a transgendered man who cannot/will not get hormones
He was born Henrietta unfortunately, which explains all of the issues with femininity he has (for example, high pitched voice, no facial hair, being forced to wear dresses, etc.) This was also why his parents always mistreated him. They wanted a son and not a daughter. His father, before having Roger, named his dog Onlyson because he didn't think he would ever have one. When Heinz came out to his parents, his father forced him to do tests like jumping off the high dive and, when he failed these tests, ordered him to be a lawn gnome as punishment. In the end, Heinz moved to America and was married and had chest surgery, but for some reason couldn't/wouldn't get hormones. His wife had a child using a sperm-doner and he raised her, but eventually Charlene wanted a more "normal" marriage and they broke it off.

Doofenshmirtz is a troper
Who says we wrote the intro to the main WMG page to look as if he wrote it? Maybe he actually DID!
 * He does lampshade stuff often...
 * (OP) So which troper do you think has the highest likelihood of being Doof?

Major Monogram is a Robot
Hey, he did say he was 35% metal.
 * Wouldn't that make him a cyborg? Well, I guess it would depend on what the other 65% is.
 * Or he was in an accident which caused him to have robotic limbs.
 * So does he also have a grudge against all robots, and they're the reason why he keeps sending Perry after Doofenshmirtz?

Major Monogram used to be a secret agent.
In "Summer Belongs To You", Major Monogram stated that he's part metal. Perhaps he got that from an accident when he was a secret agent.
 * Aren't all of O.W.C.A.'s secret agents animals? (Well, besides Planty the Potted Plant.)
 * Major Monogram being a secret agent doesn't necessarily means that he worked for O.W.C.A.. Perhaps he used to work at another institution.

Monograms singing personality is his repressed homosexuality
He wears pink, has blond dyed hair, acts really feminine, and Monogram has "Tried to keep [singing-side] hidden for years". Monogram has tried to hide his sexual orientation because he feared it would make him seem less masculine.

Major Monogram is the true Big Bad of the series
Or at least, the closest thing to a main villain the series has. He always sends Perry, one of the most competent agents of the organization to fight Doofensmirtz, a Harmless Villain. Clearly(or not), he's building up his evil in the background.

Major Monogram is Isabella's father.
Major Monogram's wife does sound Mexican, and there's a hugging scene between Monogram and Isabella in "Doof Dynasty".
 * Perhaps Major Monogram is Isabella's uncle.
 * Jossed in the family christmas special. He was not shown as part of Isabella's clan on either side.
 * Her father didn't appear in the clan either... or did he?
 * Isabella's mother is front and center on the mother's side of the lineage. On the Jewish side of the lineage, a man in a brown suit was in a similar formation (front row on the left) to Isabella's mother. That man is Isabella's father. She got her hair color from her mother.
 * If he's Isabella's father then he would be standing next to her mother. Then again, Major Monogram didn't stand next to Isabella's mother either...
 * It's possible they did it that way to show that half of her family is Jewish and the other half of her family is Mexican.
 * Nah, Major Monogram is probably Isabella's stepfather, since Isabella didn't call Major Monogram "Dad". Plus we didn't know how did Isabella knew Major Monogram's name. It might also be the reason why Isabella and her mother's last name isn't "Monogram".
 * If that was the case, then Isabella wouldn't have had to lose her memories at the end of "Across The Second Dimension".

A theory on the various kids' ages.
We know for certain Candace turned 15 in "Candace Loses Her Head," Vanessa turned 16 in "Dude, We're getting the Band Back Together," and Jeremy is "a whole year older" than Candace according to "Across the Second Dimension." Phineas (post season 3) and Ferb themselves are heavily implied to be 12 in the Family Christmas Special. The rest of the characters' ages are up in the air. Here are some theories based on the scraps of evidence the creators have thrown us.
 * Isabella: 12-13. Almost certainly the same age as Phineas. Her leadership of the Fireside Girls supports this. Girls tend to develop emotionally a bit quicker than guys as well, so that explains her crush on Phineas, but his not entirely reciprocating yet. He still likes her a lot, but he's not wired for romance quite yet. She is most likely older than Phineas, perhaps the difference of several months.
 * Buford: Almost 13. He was the hardest one to place. Some episodes (like "Voyage to the Bottom of Buford") make him seem younger, just a bit overgrown, but that could be a problem with mental/emotional development. I decided to skew a bit older to play it safe.
 * Baljeet: 11 going on 12. He's younger than the other members of the group, but definitely around their age enough to be unsupervised so much and to be a regular part of their activities.
 * Mishti: 11 or 12. She's never implied to be anything other than the same age as Baljeet. Though she "becomes a girl" between when Baljeet came to America and when she shows up in "That Sinking Feeling", it's not unheard of for puberty to start at around that age in girls.
 * Django: 10 or just turned 11. Definitely younger than his friends, and probably the youngest of the group. It also explains why he's not around much since he most likely is still around the age of needing more parental supervision (that, or he tends to hang around with more people his own age).
 * Albert: 19. His know-it-all attitude, the fact that he goes away to school, and his voice imply that he's off in college, but also that he's still in the 'lord it over the younger people' phase of development into adulthood.
 * Albert is said to go to 'boarding school', which implies that he's less than college age. If he went to college, Irving would have said that. College is not boarding school.
 * Irving: 11. The way he obsessively looks up to Phineas and Ferb suggests that he's younger than the others, and the way they don't tell him to leave because it's creepy supports that to me.
 * Stacy: 15. They're in the same grade according to Stacy's reminiscence in "Spa Day," so they're either the same age or Stacy will turn 15 within the next couple months (assuming neither was held back or started late).
 * Coltrane: 16. We haven't seen much of him, but he seems to be the same age as Jeremy.
 * Jenny: 16. Her mellow hippie attitude makes me think she's older than Candace and Stacy, but not by much.
 * Carl: 22. He became an intern soon after the flashback to 5 years ago in "Across the Second Dimension," so he must have been nearly college age then.
 * Gretchen: 11. Though she's Isabella's second-in-command, she seems younger, but her knowledge makes it hard to judge her age, so I'm putting her at the middle of the pack.
 * Since when is Gretchen second in command? Adyson was in charge when Isabella was gone in "Summer Belongs to You."
 * Adyson: 12. Definitely at least as old as Isabella, but most likely not older since then she'd probably be troop leader instead.
 * Ginger: 11. Looks younger than Adyson and Isabella, but older than Holly and Milly.
 * Katie: 12. She looks about the same age as Adyson and Isabella.
 * Holly: 10. This is mostly just because she looks significantly younger than most of the other Fireside Girls.
 * Milly: 10. This is mostly just because she looks significantly younger than most of the other Fireside Girls.
 * Suzy: 8. Definitely at least a couple years younger than anyone else in the cast, but she's definitely older than 6 for the sheer amount of scheming and knowledge under her belt. She also definitely still needs to be babysat unlike the other kids.
 * Since she's implied to be wise beyond her years, it wouldn't really be out of the question for her to be sixish. Her inarticulate speech (when she's not talking to Candace) suggests a very young age.
 * Johnny: 17. He's analogous to Vanessa as Jeremy is to Candace, but since she's a year older, I decided to place him there, too.
 * Marty the Rabbit Boy: 18. He seems calmer and older than the other teens aside from Jeremy.
 * Unknown Guy: Who knows?! Unknown guy is out. Peace!
 * Actually, Phineas and Ferb are 10. In that episode where they go 20 years into the future, their mom says,"My Phineas and Ferb are 30 now..." What's 30 minus 20? 10. There ya go.
 * It's been said by the creators that they had to give a relative age to them for the purpose of the episode and to not to take Linda seriously. So, the "official" age for them is "less than 15".
 * I'd just like to say I highly doubt Phineas and Ferb and their friends are as old as they're guessed to be above. They (especially Phineas) have the optimism and lack of self-consciousness that you'd expect from children who are still grade school-aged. Linda's attitude toward the boys should also be considered. She thinks they play elaborate pretend-games and tell her about them as if they were real. If they were as old as 12 and 13, she would probably find that behavior weird instead of cute. I'd guess Phineas is 9 or 10, possibly 11.
 * Phineas was 9 in the original pitch, for what that's worth.
 * I suspect the key to the puzzle is Isabella. We know that Phineas and Isabella never met in the alternate dimension after Doof 2 took over. This event was sometime after first meeting Perry (which was 5 years ago). Isabella's brain is currently wired for romance, but Phineas' has not done so yet. Knowing what we do about her future, the Westermarck Effect did NOT take place with neither Phineas nor Ferb, meaning that she first met the boys after they turned 7. Given the difference in brain wireing and that she's taller than Phineas (remember gender growth rates and times), this leads to two likely conclusions. 1. Isabella is most likely older than Phineas (maybe a few months at most), 2. The starting age of Phineas can be narrowed down to 11-12, with a season 3 +1 modifier to it.

A lot of the antagonists (the word antagonist in this situation is used very loosely) can be described as having a fear of loneliness.

 * Heinz Doofensmirtz: Heinz Doofensmirtz grew up in a situation where he had little to no friends and a sucky family. When he was a kid his best friend was a balloon he had drawn a face on, while in the present day he has made use of his nemesis relationship with Perry the Platypus in order to have someone to hangout with on a daily basis. Although Perry often foils his schemes, deep down Heinz just enjoys being able to hangout with someone. Perry realizes this and hangs out with Heinz even when he's not up to anything evil. On the family side of things: Heinz's parents treated him like crap, he doesn't get along well with his brother, and his wife divorced him. The only family member he has left that still cares for him is his daughter Vanessa. Sure Vanessa is embarrassed by her dad at times, but she still cares for him. The reason why Heinz is so nice to his daughter is because he doesn't want to lose the one family member he has left (in a manner of speaking), because if he lost Vanessa he would be left with no friends (not counting Perry) and no family to depend on. In short, Doofenshmirtz just wants to have friends and a family that actually cares about him.
 * Candace: As shown in the Ducky Momo Is My Friend song, Candace seemed to be very lonely when she was little with her only friend being a stuffed toy. Eventually her brother Phineas was born, and as he grew older he and Candace got along well with each other as Candace finally had someone she could actually interact with. Eventually however Ferb became a member of the family and Phineas started spending more time with him than Candace. When Candace saw the amazing things her brothers were capable of her loneliness set in again as, although she really wanted to help, she felt that there wasn't anything she could do. Causing her to ignore her brothers asking her if she wanted to help (something they continue to do in the present day). Eventually her loneliness turned into jealousy especially when she realized that her brothers were capable of getting away with all these amazing things, but she wasn't able to get away with more minor stuff. This led to her busting obsession as she subconsciously decided that if she couldn't reach her brothers' level then she would knock them down to her level, and they would be able to hangout with each other again. Her loneliness turned jealousy however has left her blind to the fact that she is able to participate in and have fun with her brothers' ideas. Candace however has forgotten her original reason for wanting to bust her brothers, and tells herself it's because she's in charge of them and that they shouldn't be doing those things. In short, her obsession with busting her brothers comes from a subconscious desire to hang out with them.
 * Related theory on how Candace and Stacy became friends: When Stacy was little she and her mom lived in Japan. However when her mom got a job offer in the USA, Stacy was forced to move away from her life in Japan. For obvious reasons her mother had her take English speaking lessons. When she entered her new school in Danville she was shy and nervous, because not only was she going to a new school but she was in a different country with a different culture than the one she was used to. Candace recognized Stacy's loneliness and decided to befriend her. With Candace's help Stacy was able to come out of her shell resulting in the Stacy we know today. Candace and Stacy instantly became best friends as Stacy provided Candace with a friend who wasn't a fictional character or her brother, and Candace helped Stacy get used to her new life in Danville.
 * Buford: When Buford was younger he didn't have any friends. When he won his pet goldfish, Biff, at a carnival he was happy as he now had a friend. However when the local bully attempted to eat Biff, Buford became a bully himself in order to protect Biff. Deep down however Buford realized that Biff just wasn't the same as a human friend, causing him to develop anger issues that helped out his new career as a bully. This led to him making Baljeet into his own personal nerd as an excuse to have someone to hangout with (similar to Doofenshmirtz's relationship with Perry). When he first met Phineas and Ferb he considered them to be a couple of nerds, however in Raging Bully he gained respect for them and started to hangout with them more often. But it wasn't until Voyage to the Bottom of Buford that he started to realize that Phineas, Ferb, and the others were his friends and he started to mellow out around them. It should be noted however that Buford is still technically a bully meaning there's a small part of him that is afraid that the others don't see him as a friend and that Biff is still his only friend (an assumption he would be wrong about). In short, Buford just wants to have friends and is afraid that the friends he now has might leave him.
 * Buford is not an antagonist, he never tries to stop the boys and frequently even helps them.
 * Well I did say that I was using the word antagonist very loosely. I am aware that calling Buford an antagonist is a bit of a stretch, but I just thought that I should include him since he kind of started out as an antagonist (sort of).
 * Suzy Johnson: It's possible that like Candace and Buford when they were younger, that Suzy doesn't have any friends. Her brother Jeremy, however, is always nice to her causing her to develop an attachment to him. But because she's so young she doesn't quite get the difference between a relationship with a girlfriend and a younger sister, and so she purposely scares off any girl she thinks might take Jeremy away from her (although she seems to be failing with Candace who is afraid of her, but manages to get around that obstacle in order to be with Jeremy). As for why she scares Buford, it's probably just for fun. In short, Suzy attachment to her brother Jeremy is because she doesn't have any actual friends.

On a similar vein to character's ages, anyone care to guess Birthdays?
Phineas, Candace, Dr. Doofenshmirtz, Vanessa, Linda, and possibly Perry have summer birthdays, but most of the dates are a mystery.
 * We do know Candace's birthday is in July 11th.
 * Jeremy's birthday is probably between January and July, since Candace says "he's a whole year older than me" (rather than "almost a year" or "a year and a half"). There's also a good chance it's not in May-July, since he hasn't had a birthday episode (although the achronologic order of the episodes means we can't rule that out).
 * Over here is a guess that Doof is a Gemini. Seems legit.
 * In "Dude, We're Getting the Band Back Together", Candace mentioned that it was June 15. Doofenshmirtz spent that day setting up a birthday party for Vanessa.

Sergei the Snail is Sergei the Astronaut's pet.
His child named Sergei after his father.

Constance ended up marrying Phineastein

 * Constance was referred to as a disapproving governess and she looks like Candace while Phineastein looks like Phineas although they are not mentioned to be related to one another, so this theory explains why they look like Phineas and Candace who are siblings.
 * But Constance was obsessed with Jeremiah. It would be terribly out of character.
 * She may have had a crush which she could have easily gotten over it.

Princess Baldegunde is a descendant of Constance

 * The story in "Phineas 'n' Ferbenstein" is implied to take place in Druelselstein. Baldegunde is from Druelselstein, and resembles Constance and Candace. Do the math.

The members of Linda's line are all obsessive on the subject of 'busting.'
Look at 'She's The Mayor', 'Candace Gets Busted', 'Quantum Boogaloo', and even the All Just a Dream story in 'P&F Get Busted'. Linda knows that her family has this tendency. She is actually afraid to see the boys get busted, knowing she will badly overreact, hence she self-inserts endless delays into Candace's attempts and belittles them to try and push her daughter away from this potential apocalypse trigger habit. Phineas has it too, but he wants to 'bust' the rules that say you can't do this or that, and manages to avoid causing harm, since this is his joy and not a nervous tic. This is why Linda is so zero/one on busting her kids.

Candace and Phineas both have OCD.
Phineas is happy because he has his life completely in order. His compulsion drives him to build things (which is why he freaked out when he was on an island and found it was impossible to build anything). Every day, he utters all the same catchphrases, much as people with OCD perform rituals. After being told by Ferb that he was "not very street", he became sad...and then he said "Hey, where's Perry?" and immediately cheered up.
 * People with OCD engage in repetitive ritualistic behavior that brings them no pleasure (besides the temporary alleviation of the anxiety that plagues them when they aren't performing the rituals they believe they should be). That doesn't sound like Phineas at all.

All of the characters made a secret vow to never talk about the giant floating baby head after its first appearance.
They were so creeped out by it when they first saw it that they decided it was better to just ignore it if it were to ever show up again.

Phineas and Ferb's family are eccentric millionaires
Come on! They can afford incredible expensive and complicated projects; how do they pay for them? Simple: They are millionaires!

How? Linda Flynn's hit single and its copyrights (the song is played in most elevators of the city).

Nonetheless, they want to live the "simple life" of the suburbs.
 * That's not where the money for the stuff comes from. Phineas and Ferb just have a bunch of credit cards under various fake identities. In fact, they're responsible for the recession.
 * They can afford a pet platypus, and they have enough money to get around the laws that are against that sort of thing.
 * Well, it was revealed in the recent episode "Oh, There You Are, Perry" that Perry's only one of many platypi in Danville. Maybe Danville's some kind of resort town for eccentric rich people.
 * Considering in The Lizard Whisperer it shows Phineas, Ferb, and Isabella running through town without even their sister watching them, it would prove the idea that they pay for the town to be safe (you know, the Organization Without a Cool Acronym).
 * Another strong possibility is that Phineas and Ferb hold some profitable patents, like the MIB.

Thaddeus and Thor will come back for revenge.
They just don't strike me as the kind of characters who would take their defeat lying down. (Or Thaddeus doesn't, anyway.) As soon as they get over their BSOD, they'll be out to get Phineas and Ferb for humiliating them.

If you need further evidence for a return, Thor still hasn't said anything. An episode in which Thaddeus and Thor recur would allow the creators to give him a Ferb-like one-liner, complete with ridiculous accent.
 * Are you calling Ferb's accent ridiculous?
 * Read on a blog that Thaddeus and Thor are in an upcoming episode. -OP
 * You mean "Perry the Actorpus"? I thought that was only Mandy.
 * If Mandy is in it, it seems reasonable that her brothers would get at least a cameo. So far her character exists only as a foil for them.
 * Nope, they didn't appear.

Prince Charles and Princess Baldegunde will become an item.
Royal doubles of Jeremy and Candace? Frankly, I'm surprised they didn't end "Make Play" with a Call Back gag involving Charles. I have no doubt that somebody on the writing staff will pick up on this idea and run with it, possibly in some kind of epic parody of the Royal Wedding.

Danny, Bobbi Fabulous and Sherman/Swampy are Timelords.
It makes sense! They sang in Doofenshmirtz's flashback (and knew it was a flashback), they somehow managed to get to the Second Dimension to sing Robot Riot without Phineas, Ferb, or Doof's knowledge, and in Rollercoaster: The Musical! they all reverted back to the ages they were when they were first popular. How ELSE do you explain it?!
 * They didn't go to the second dimension, the Robot Riot montage happened in the first dimension.

Dr. Diminutive is Irving in disguise.
He wants an excuse to hang out with the mad scientists when he's not stalking Phineas and Ferb.

The Phineas and Ferb expies seen on an episode of Mad are really Thaddeus and Thor in disguise.
Cause Phineas and Ferb wouldn't be such jerks in reality! Also, they're trying to outdo the "Straight A Team", just like Thaddeus and Thor! Remember?

There's another secret organization run by Wanda Acronym's far cooler relative.
Which is why the OWCA is the "Organization Without a Cool Acronym."

Charlene is Poofenplotz's Daughter.
Why else would she have kept the last name Doofenshmirtz?

Stacy sometimes uses Phineas and Ferb's daily projects as a excuse to hang out with her little sister Ginger.
It makes up for the fact that they aren't normally seen together.