Just Add Water

In a video game with an Item Crafting system, everything that can be created can be made by two items combined in a relatively simple fashion. No matter how complicated something is to make, sticking the right two pieces of Vendor Trash into an oven will produce it. Need a BFG? Just glue a tiny hammer to a hollow tube, and you're ready to go! Forget your Love Interest's birthday? Pop an egg and a bag of sugar (still in bag, mind you) into the oven, and you'll have a delicious and moist cake!

Occasionally, advanced items will require multiple iterations, but even in this case, every step in the process requires sticking two items together, and usually the intermediate steps produce items that are usable in their own right.

Similarly, chemists and potion makers can create any liquid or potion by mixing two liquids in kept test tubes. There may be a chance (depending on the game) of getting a small explosion instead, which can have any effect from simply coating the face with a harmless dark, dusty substance, to changing the potion maker into an animal.

This may or may not be an Acceptable Break From Reality (depending on how far they stretch it).

Not to be confused with Instant AI, Just Add Water.

Examples of Just Add Water include:

Action Adventure

In The Legend of Zelda: The Wind Waker, a potion maker can make any of his wares from a single type of Chu jelly, though he needs several units for a full batch. Possibly justified as it he might be simply distilling or refining the jelly in some way. In Twilight Princess, you can drink the jelly straight for the same effects.
Dead Rising let's you make all sorts of incredibly destructive weapons using random things found around the game world and liberal use of a seemingly infinite amount of duct tape.
- If you've ever watched The Red Green Show, this may be Truth in Television.
- However, you only need two items, yet the result features more. For example, a Paddlesaw is made from a canoe paddle and a chainsaw, yet the final result has two chainsaws.

Adventure Game

Subverted in the obscure adventure game Muscarine where combining blue snow and yellow snow is not how to get green snow. (You still have to try to receive a hint about the proper method, though.)
Minecraft lives and breathes this trope. Items are assembled by actually more or less drawing what you what with the requisite materials. A torch is a stick with a lump of coal on top, a bookcase is wooden boards with books in the middle, a pickaxe is two sticks end to end with three blocks of the head's material (wood, stone, iron, or diamond) crosswise at the top.
- Cooked Pork and Cooked fish require a heat source to be created. Bread and Cake do not.

Fighting Game

Power Stone 2 lets you mix 2 items to get a new one. Obtaining multiple copies of ones you already own would make the chances of it appearing during a fight higher.

First-Person Shooter

Team Fortress 2 has a large crafting system where one can take unwanted items and smelt them into scrap metal, which then gets combined into reclaimed and refined metal, which then can be made into the game's ultimate goal. Specific recipes also exist to craft specific other items and weapons without having to wait for the random drop system to give them to you.

MMORPGs

In Kingdom of Loathing, you can make food, drinks, equipment and even living creatures by combining two items. This is parodied with a few things, such as the item Dry Noodles being able to be almost any kind of pasta (cooks to make lasagna, spaghetti, ravioli, ect). Supertinkering requires three items but is otherwise the same.
- The Pasta Crafting is justified by the fact that you are a pasta specialized magic user.
- "Justified?" Dude, this is a game where you can make a superweapon by attaching a toilet plunger to an engine block, and a pair of Sweet Rims is a required part for all cars.
- Cottage cheese + anticheese = cottage. Whoa...
- You can combine two leaflets (i.e. pamphlets) to make a leaf. Which you can then combine with maple syrup to make a maple leaf.
- The Untinker can un-combine pairs of items with a screwdriver, even though items are combined using meat goo or plunger suction, not screws. An abridged dictionary disassembles into a regular dictionary...and a bridge. Don't try to make sense of any of this.
On Neopets, there is a cooking pot where you can combine up to three items to make things.
Webkinz has several different cooking items. All of them use three-ingredient recipes.
Averted in Puzzle Pirates. Crafting alchemistry items involves mixing primary colours to make secondary colours, then piping the secondary colours to their containers. In multiple iterations, typically.
A lot of equipment can be made with just a base item and several stacks of the needed minerals in Ace Online. However, you do need to use the Factory (Or the Laboratory for enchanting/gambling weapons) in Arlington or Bygeniou city to assemble those items.
Averted with World of Warcraft's later Engineering recipes, where you have to build the parts first; If you break it down into the smallest pieces, many recipes take upwards of 10 parts.
- While usually played straight with everything but engineering, special mention should be made of cooking, where they decided that two parts was too complicated and removed spices from the game. You can now make a spicy dish with one piece of meat and nothing else.
- However it is a bit of a head-scratcher that almost every crafting profession requires copious amounts of odd ingredients, but none of the alchemy recipes require water to mix the herbs in. Add two dry herbs in a flask and poof! A drinkable solution.
  - Shush, they'll hear you.
Averted in EVE Online. Even the simplest manufactured items are just that - manufactured. You get the minerals (from mining, the player market, refining etc.), a blueprint and bake them in a production slot. This may take anywhere from a second to a few weeks, in the case of supercapital ships. Also, the capital ships and T2/T3 ships need more components - which have to be built from minerals, reverse engineered from ancient relics (well... ancient AI spaceships you destroyed and tried to analyze a bit) and some parts can only be bought on the NPC market (which works very differently from most MMOs - while the supply itself is infinite, it comes in a steady pace. And the more there is in a station, the cheaper it is. And every station in the galaxy is covered by some PC merchant who buys them when the price is right...). Not to mention the research you have to do upfront to even be able to produce it (in the case of T2/T3 ships) and in some cases to actually make profit (production time and efficiency research). And did I mention the skills you need to build the advanced ships? And you can't rush it either - they train in real-time, just like all the other skills in the game. All in all, if you start a carrier as an industrialist, you still may never ever get to make a single mothership. And a single mothership sold can easily yield you enough in-game money to play the game for ten years without having to pay the subscription.
You rarely get to see what you're actually making in City of Heroes, some of the recipes are... odd... Some examples:
- Inanimate Carbon Rod + Boresight + Shiny Ring = Piston Boots!
- Mathematical Proof + Gold Brick + A Fortune + Sapphire + a chunk of Unobtanium = A Fusion Generator! And you only need one of each item!
Notably averted in EverQuest 2, where crafting operates somewhat like the combat system: various trouble spots and opportunities for bonuses pop up, requiring you to react by using the correct ability. You can end up with a junky or masterwork version of the item depending on the results.
Mabinogi averts this in that foods and items are crafted through somewhat complicated processes and require a skill minigame of sorts to finish.

Platform Game

Kirby 64 plays straight though with ability-making: You can combine only two "elements" to make an ability.

Puzzle Game

In a similar vein, the entirety of The Secret Island of Dr. Quandary is based around collecting various items (such as ABC gum) and mixing them all together in a giant magical cauldron so that you don't have to spend the rest of your life forced to solve math puzzles while trapped inside the body of a troll doll.
The Android App game "Alchemy" is nothing but this trope. You start with nothing but the four classical elements—earth, air, fire, and water—and by combining them, you can make anything from mud, to people, to continents.
- On a similar note, the games Doodle God and Doodle Devil for the iOS.

Roguelike

Nethack has several:
- Pouring one potion into another has a tendency to create a third potion.
  - Or explode, of course. Because why wouldn't two different doses of healing potion explode when you mix them together?
- Dragon scales (dropped by dragons) + Scroll of Enchant Armor = Dragon Scale Mail (one of the best armor choices)
- Once you know a scroll or spellbook, you can write more copies with a magic marker on a blank scroll or spellbook. On the other hand, magic markers are hard to come by and often have to be wished for.

Role-Playing Game

In Dark Cloud 2, you "invent" things (most of which have long since been invented) by taking pictures of random stuff, and combining them for inspiration. Then, you need a certain number of several raw materials to make it, and there you go!
In Star Ocean the Second Story, there are only six ingredients (meat, seafood, eggs/dairy, fruit, vegetable, grain), but with just two of them, you can make anything. You can also cook items like shark fin soup or cheese pizza with one ingredient.
Alchemy in Dragon Quest VIII
Every single one of the recipes in all of Gust's Atelier games. The entire series of games is based on this trope. Some games force you to use special utensils for certain recipes, but only until your Alchemy level is high enough.
The Paper Mario games. In fact, you don't even start out with the ability to cook with two items! Just one.
- In the original, Tayce T. can only cook with one ingredient at a time, but after you gain a Cookbook (a very likely possibility in the normal course of a game) and give it to her, she can cook with two.
- In Paper Mario: The Thousand-Year Door, Zess T. can only cook with one item at a time until and unless you retrieve a cookbook; it's easy enough to get to, but it's not like it practically falls into your normal path the way the first game's does.
- In Super Paper Mario, Flipside has a chef who can only cook with one item at a time; the chef in Flopside, on the other hand, can cook with two.
The cooking of poffin in the Pokémon Diamond and Pearl games: berry + mixing = ...bread? Though, to be fair, there is visibly batter in the mixer before you throw the berries in.
- Less excusable is Pokéblock creation in Pokémon Ruby and Sapphire, berries+centrifuge=... a fruit cube?
  - Ever seen fruit leather? It could well be made in a machine similar in fashion to them, drying and compressing as it goes.
Usually averted in Final Fantasy XI: Most recipes, while maybe not including every last little spice, come across as fairly realistic in terms of ingredients. What isn't averted is justified, as crafting as PCs go about it is an essentially magical process.
Arcanum has this for all of its inventions, although some items may require a chain of creations: say, iron ore and steel to make pure ore and pure ore and a handle. Notably, it does this in violation of conservation of mass, where not only can you lose weight from it (which makes some sense) but you can also gain weight.
Wizardry VIII. Moreover, two source items, required skill and its value are moddable properties of item. Engineering (Gadgeteer class) and Alchemy (Alchemist class) skills are widely used that way. You can get extra profit from Alchemy combining 2 cheap item into 1 expensive or at least turn 2 heaps of rather pitiful items into 1 heap of really useful ones.
Aversion: Fallout 3 has Four Ingredient "Cooking".
Averted in The Elder Scrolls: a basic potion can be made with just two ingredients and a mortar, but complex potions can be made with four ingredients, mortar, retort, alembic and calcinator.
- Daggerfall evades the question altogether by only allowing people who belong to a Temple to bring ingredients to a potion-maker.
- But in Oblivion, once you get that 100 Alchemy (not that hard), all you technically need is one ingredient and a mortar. Just Add Water indeed.
  - Though making potions from one ingredient is only slightly better then simply eating the ingredient.
  - Could be played straight, could be averted, depending on your point of view, with the in-game scrolls and notes that when read contain alchemical recipes: Every single one, without exception has "one flask of water" or some equivalent in the list somewhere.
Averted in The Witcher: In order to create an alchemical mixture, you usually need at least more than just one herb/mineral/monster drop in order to get the substances you need to make the concoction, with more elaborate formulae requiring rare "special ingredients" alongside them. Furthermore, you'll actually need a liquid or powder base to mix the ingredients with.
Alchemy in Odin Sphere involves adding something (usually a type of living plant) to a vial of numbered liquid to create a potion. There's also a variation for cooking in the Pooka village restaurants.
Knights of the Old Republic II had a system where you could break down items into "components" and then rebuild them into other items, leading to delightfully bizarre scenarios where you could turn a bunch of swords into a suit of armour. The same was true of chemicals, which could lead to disassembling a poison gas grenade and using the chemicals to build medical supplies or cybernetic implants.
One of the town upgrades in Breath of Fire 2 is a restaurant of sorts, where you can combine up to 4 items together, often with extremely implausible results, such as making gold bars from spices and soup, octopi from worms and roast beef and frisbees from curealls.
Monster Hunter is a prime example of this trope, though some combinations require either higher levels of Books of Combinations, or a certain armor skill to reach 100% probability of mixing. Some items can only be gotten by combining other combined items, and there is actually an achievement for combining everything that can be combined at least once. It really starts to get weird when you get into the Alchemy mixes, though. For some reason, wearing the right armor can allow you to combine a burnt meat and a piece of garbage to make a raw meat...
Online time-killer Game! has two major game sections that run on this trope. You can combine items with duct tape to create more useful items, or try your hand at cooking.
Crisis Core: Final Fantasy VII's Materia Fusion works this way. Slap together two Materia, perhaps some items if you have the right key item for it, and you have a new Materia! This is the only way to max out Zack's power. The sad thing is that with how little bonus you actually get out of normal leveling, and how damn powerful the mooks eventually get, you really need to spend some time with this.
Ring assembly in Lost Odyssey. Need a Jamming Ring? Just stick together 5 "Junk Parts" and 2 "Whetstones". The components are often bizarre, from rocks and ores to Magitek devices to monster body parts, even including a certain monster type's souls. As usual, the more advanced rings require their lesser versions as components.
Used literally in Dragon Age, where the ingredients for a simple poison are... Venom extract and water.
Grathmelding in Ar tonelico 1, where you combine a magic crystal and another material to get items (which can then be crafted into other, fancier stuff). Generally justified as the game shows Lyner working the materials through various processes (and implied to include other less exotic materials) and explaining how the component items contribute. Aurica plays this more literally by borrowing two food items from your inventory and sticking them together to create "amazing" new recipes like barbecue meat soda.
In the Mega Man Legends games, Roll can build any number of weapons for Mega Man just by combining odd pieces of junk. Even books and toy guns!

Simulation Game

Averted in Harvest Moon: Friends of Mineral Town. Not only can you use up to eight ingredients in cooking (not counting spices), but you also have to choose the right utensils to make the recipie! Luckily, most of them are fairly intuitive, except for the ultra-special recipies.
In Zoo World you can build a rose garden simply by gathering five roses of each of the primary colors. To level it up you need to gather additional roses of additional colors, but no sweat - roses of secondary colors can be created by combining primary colors.