Insert Grenade Here

This is where a tank is on the battlefield and the main protagonists have either used up all their anti-tank weapons or never had any to begin with. Fact of the matter is that modern-day tanks can take repeated RPG hits and the crew inside will probably be fine (one Challenger 2 took as many as 70 hits in 2003 and the crew only suffered one broken wrist).

You run up to the tank (hopefully nobody sees you) jump on, kill the guy looking out of the hatch or open the hatch (it's never locked -- just in case the crew does need to get out fast) and then toss down a few grenades before jumping clear of the resulting explosion. Often last part of the plan doesn't work making it a Heroic Sacrifice.

Should be noted that this was more or less the original use of the Molotov Cocktail. In that case, though, you threw them onto the back deck in the hope that the burning gasoline would enter the engine compartment through the air intakes.

Anime and Manga

 * In the very first episode of Madlax, the titular Action Girl takes on a light tank, tricks the driver by "disappearing", then when he opens the hatch to look for her, kills him and tosses a grenade down the now-open hatch. And that's just one of the impossibly awesome things she does during that battle sequence.
 * Subverted in Ghost in The Shell. At the climax, the Major faces off against a Spider Tank using only an assault rifle. She manages to jump up on top of it, but
 * Played fairly straight in the beginning of Appleseed, where Hitomi takes down Bluebeard and his crew while they're busy trying to kill Deunan and Briareos.
 * King Bradley in Fullmetal Alchemist pulls this, but instead of pulling open the hatch he breaks a hole in the viewing window then shoves the grenade through there.

Comic Books

 * This was a favorite tactic of Sgt. Rock.
 * In Nth Man, White Ninja John Doe manages to Insert Grenade Here by throwing a grenade down the gunbarrel of a tank that is about to shoot him, timing it exactly to match the tank's gunner opening the breach. He manages this feat by a combination of good hearing, good timing, and channeling his memories of zen concentration from playing baseball with his adopted father.
 * In G.I. Joe Special Missions #3, Leatherneck expertly pops a grenade from his underbarrel grenade launcher in through the open viewport of an APC. This being the comics and not the cartoon, it almost certainly kills the crew.
 * Sgt. Nick Fury did it, of course. See here.
 * Kato does it to a tank using a smoke grenade in an issue of Dynamite's Green Hornet series. Probably intended as a Shout-Out to the Live Action TV example below.
 * The Black Rider does this to an APC in Marvel Comics Six Guns mini-series.

Film

 * In Red Dawn when the group is pinned down by a tank (which doesn't see them) one of the main characters attempts this but gets hurt pretty badly and is unable to complete the task, instead he sets off a smoke grenade so that friendly fire can spot the tank and blow it (and him) up.
 * Occurs earlier while the tank is refueling, and works considerably better.
 * That was more a case of setting them up the bomb.
 * In Saving Private Ryan, the Airborne soldiers trying to hold a bridge don't have much in the way of anti-tank weapons, but are faced with an armored convoy. To fight them, they come up with a Sticky Bomb: pitch-covered socks filled with explosives, designed to be slung onto tank treads from close range to disable them and block the road. Suffice to say, the results of the sticky bombs are... mixed. (This was Truth in Television, including the fairly "mixed" results.)
 * They also do the traditional 'climb on top and bung a grenade down the hatch' version once the sticky bombs have disabled the tank - it works perfectly, apart from the fact that there's an anti-aircraft gun pointed at the tank, which quickly shreds the soldiers who took it out.
 * Sometimes done in Delta Force movies, one of them is done using a Molotov Cocktail.
 * A variant on this in the movie version of Starship Troopers: Johnny Rico jumps onto the back of a giant bug, blows a hole in its carapace with his gun, and then tosses in a grenade. Much orange and green paint ensues.
 * In Star Wars Luke does this to an AT-AT by grappling up to the underside and cutting a hole in the bottom with a lightsabre.
 * Jar-Jar does a variation in Phantom Menace with one of those weird electronics-killing water balloon things the Gungans have.
 * Battle of the Bulge (1965). Lt. Colonel Kiley (Henry Fonda) shoots the commander of a German tank, climbs up the tank and drops a grenade through the open hatch, killing everyone inside.
 * The same thing was done in Defiance, Zus arrived with The Cavalry, climbs up from behind the tank, guns down the German tank commander, and drops a grenade into the hatch.
 * Tobruk (1967) cleverly put this together with Fake in the Hole for an ingenius German tank heist. Antiheroic Maj. Donald Craig and his British squadmates empty a hand grenade for use against an approaching Nazi Panzer. Then, they approach the vehicle, kill its machinegunner, popped the hatch open and tossed the dud grenade in. In panic, the rest of the Panzer's crew climbed out of the hatch, only to meet the waiting submachineguns of Craig and company.
 * Subverted in All The Queens Men - at the start of the film, a Nazi tank is pursuing a double agent that hijacked a second tank. The Nazi tank fires a few shells at the agent's tank, and it stops - one of its crewmen then jumps out of the tank and runs over to the hijacked tank, throws a grenade in, and fires a few bursts into the crew quarters with his submachine gun. He then enters the crew quarters and after a few seconds comes out with a confused look on his face - just in time to see the double agent ride off in the other tank.
 * In Avatar, Jake does this with Quaritch's gunship, throwing grenades onto its outer vents. It's not a straight tank example, since it's a flying vehicle, and Jake gets on top of it by means of his Giant Flyer.
 * Captain America performs this on a three-story tall HYDRA tank, following it by jumping off of the tank as it explodes.

Literature

 * In the Gears of War novel, Aspho Fields Helena Stroud does this to help out the air-support but gets snagged on the gun turret while jumping away and ends up getting blow apart.
 * The Animorphs take out a tank this way when time-jumped back to D-Day. Since they were birds at the time, they even managed to bypass the "kill the lookout" step.
 * Mack Bolan does this with a tank in Cambodia. The hatch is locked so he has to clamber up the main gun and drop the grenade down the barrel.
 * Happens all the time in the WW 2 Sven Hassel and Leo Kessler books, as the German protagonists seldom have a panzerfaust or anti-tank gun when they need one. There are frequent references to tying bundles of hand grenades together for this purpose.
 * The Ciaphas Cain novels feature several examples. In Cain's Last Stand, he is tempted to chew out a pair of cadets who nearly got themselves killed doing it, but he let them off since they lived.
 * They also reveal they had a contingency plan in case the hatch was closed.
 * Ragnar does this in the prologue to Ragnar's Claw.

Live Action TV

 * In one episode of The Green Hornet ("Seek, Stalk, and Destroy") Kato uses a "tear gas dart", which he throws into the open viewport of a stolen tank in order to force the tank thieves out of the tank and into the open.
 * A variant shows up in Battlestar Galactica: The Plan, where Anders' resistance team come up with a tactic to lob a small bomb through a hatch, into a Centurion refueling station. Since the group is made of Pyramid players, they do the lobbing bit very well. It's dealing with the Centurions already present that they suck at.
 * It also highlights that the poor guys are really clueless in real tactics and, as Anders later admits, are just getting their ideas from movies. So, literally Hollywood err... Picon Tactics.
 * On one episode of The Unit, Jonas's father was very belatedly awarded the Silver Star for destroying a tank this way during the Korean War.

Video Games
"Mei Ling: Snake, you just blew up a tank! On foot!
 * In the first Metal Slug game, the enemy Mooks would climb onto your Metal Slug, and if not shaken off in time, they would drop a grenade in. Fortunately, it only damaged your slug and not the player.
 * Master Chief is capable of this on Halo against Wraiths and Scorpions. If he jacks a vehicle from the rear he can plant a grenade the the Wraith's turbine/Scorpion's engine grill and destroy it.
 * Or he can skip that bit and literally punch the tank until it explodes.
 * Turns out Master Chief isn't the only one who can do this: ODSTs in the eponymous game can as well. They can't punch open tank covers, though, so they just pop them open and drop a grenade in. If you don't have any grenades? They improvise.
 * Snake's first battle against Vulcan Raven in Metal Gear Solid was man against tank, so this trope was used to beat it, but without the jumping on the tank part. (One could soften the tank up first by careful Claymore and C4 placement, but that's not nearly as badass).
 * If you call your support team (other than Campbell that is), they will comment on how ridiculous that was.

Snake: ...and?"


 * When you revisit the site in Metal Gear Solid 4 Guns of the Patriots, if you call Otacon at this point, he'll mention that he once asked a veteran soldier once about how one goes about fighting a tank on foot, and was told, "Don't." He then goes on to gush about how awesome Snake was for managing it.
 * In the opening video of the original Dawn of War an Ork soldier sneaks behind a Dreadnought and manages to toss a grenade inside, blowing it up AND turning the tide of battle in the Ork favor.
 * Actually, it's more of a limpet mine that magnetically attaches to the dreadnaught, but the effect is the same.
 * The videogame Fur Fighters has a level set in WW 2 with tanks, the only way to destroy them was to blow up the bridges to get them stuck and then lob in Grenades to blow them up. With the respawning enemies that level was hell, literally.
 * It happens frequently in Call of Duty, when the theater's WWII. In the first game they precede the grenade with a few bursts from a tommy gun for good measure.
 * In Mercenaries, you can hijack tanks this way.
 * Which causes some unfortunate Fridge Logic, as you'd assume the frag grenade would probably ruin in the inside... or at least make it very, very messy. Not to mention you can take the driver's sidearm and grenade on their chest multiple times, fail to put the grenade down the hatch, and have to do same thing all over again.
 * Part of the animation involves, at least some of the time, headbutting a guy wearing a helmet with your mohawked, bare-headed idiot. You also protect yourself from the blast through the open hatch by putting a finger in one ear. Mercenaries runs on action film rules.
 * In Resident Evil 5, while technically not a tank, Chris Redfield (or Sheva) after stunning the Bioweapon U-8, has the option of shoving a frag down its throat.
 * Part of the Final Boss battle in Banjo-Tooie involves firing Clockwork Kazooies into the exhaust hatch of a Drill Tank and detonating them over its gearboxes.
 * Not quite a frag grenade, but in Prototype, Alex Mercer wrenches the hatch open with his bare hands (or just Kill and Replace a soldier to make it easier) and jumps into the tank himself...followed by gratuitous amounts of blood spraying out the hatch. None of this at all bothers him as he proceeds to drive the tank.
 * In Bomberman 64, the Red Mountain boss is Hades the War Machine, a giant robot you fight in the heart of a volcano. If you smack it in the head with a fully-pumped bomb, a panel on the front opens up for a few seconds and reveals the cockpit; tossing a bomb in there gives the player a Gold Card.
 * In Valkyria Chronicles, this is a viable tactic for taking out the reactors of Maximilian's massive Batomys tank. While it would normally take three or four Lancer shots to disable one, a soldier who moves quickly enough can toss a grenade into the hatch and blow it up in a single hit.
 * Steel Panthers: World At War has this as an option for dealing with tanks at close range. Sometimes it works. Sometimes you lose the entire infantry squad.
 * Brothers in Arms used this as the second method of destroying tanks. The first being the far more intelligent "with other tanks".
 * The second installment of the Heavy Gear mech-sim game has something like this as part of its opening. Tanks are treated as extremely dire threats to Gears as opposed to being Cannon Fodder as they usually are in Humongous Mecha titles, and after seeing a teammate go down, one pilot jumps up onto the offending hovertank, blows off the hatch, and empties his Gear's main gun into the crew cabin until both he and the tank explode. Ironically, Gears can carry grenades, so he could have just dropped one into the open hatch and jumped off...

Western Animation

 * One of the ARC Troopers uses this tactic to take out a droid tank in Star Wars: Clone Wars. However, he didn't have any grenades. So he blasted his way into the tank, then shot it up from the inside with his gun that fired explosions, and jumped out. It worked.
 * Yoda did it as well, and though he had a lightsabre, it worked even better.

Real Life

 * After the German invasion of the Soviet Union failed to produce the intended immediate collapse of the Red Army, the Germans found themselves woefully short of infantry anti-tank weapons to counter the vast fleets of increasingly powerful Soviet armour. To solve this problem the Germans produced a number of training films that mostly consisted of ways individual soldiers could jump up on a tank and disable it with whatever materials and weapons they had at hand, including hand grenades.
 * Don't forget the Geballte Ladung, which was basically a standard stick grenade taken Up to Eleven by the attachment of 6 additional grenade charges wired to the center one. It could destroy or disable light tanks through the force of its blast alone.
 * There's an example of a soviet KV-2 heavy tank taking over a hundred hits from German weapons until it stopped, only to continue moving when the infantry got closer. The crew was killed only after someone managed to climb the tank and jam a grenade through a hole in the armor.
 * The "Molotov cocktail across the engine grate" version is averted by the US M-1 Abrams Main Battle Tank. The engine runs hotter than a Molotov can burn. (This makes it a beautiful target for anyone with even the most rudimentary thermal-imaging equipment, however; win some, lose some.)
 * Luckily, the Abrams is also tough enough to survive hits there.
 * Somehow, Made of Iron just doesn't seem appropriate to describe the toughness here.
 * Then there was the jihadi who tried to suicide bomb an M-2 Bradley in Iraq, and only managed to paint it a nasty shade of pink...
 * Dedicated anti-armour hand grenades working on the shaped-charge principle (example) are becoming obsolescent as main battle tank armour improves, though they'll still make a mess of a Humvee or a light APC, but the most effective ranged anti-infantry weapon tanks carry demands someone stand up in the roof hatch and swing a machine gun around. If you can get close enough without being spotted, a well-thrown fragmentation grenade can still kill the turret crew and trash the interior badly enough to take a week to fix, though this is very far from the end of your problems; there is a reason tank crews refer to infantry as "crunchies".
 * This is also why nearly every tank out there today carries a "coaxial" machine gun - a secondary antipersonnel weapon that lets the tank stay buttoned up and still engage infantry. It's fixed parallel along the gun, and is relatively limited in where you can shoot it, and doesn't have the best visibility - hence the upper guns as well.
 * Why don't tank hatches have a bolt as well as a lock? That would mean the crew could keep the hatch shut and still vacate in an emergency.
 * They actually do. They bolt on the inside, with the external "lock" being just one of those two-lined up holes that you can slip a padlock through. It's called a "Combat Lock" for a reason.
 * Er... the "blocking detachments" of Stalin's Red Army secret police used this to good affect with tank crews convicted of cowardice - they were padlocked inside the tank, unable to escape unless they successfully completed their mission.
 * In combat, though, the last thing you want is a locked hatch. Time taken to unlock it is time you don't have when your tank is on fire...
 * Soviet WWII-era T-34 crews had to open the hatch when firing to vent the smoke from its powerful (for the time being) cannon until this problem was solved in T-34-85. Probably a lot of early tanks had similar problems.
 * Worth noting that a modern tank is much faster than it looks like it should be, some of them able to travel at or near highway speeds (the British Challenger tank can travel up to 60 mph easily). On an open battlefield, good luck hopping on one of those. Of course, in an urban setting, the tank might have to slow down and would also have the risk of being blocked in somehow.
 * Also, they are currently developing remote-controlled gun turrets which the commander can use to defend the tank without opening the hatch, and that's before you consider that tanks can carry Canister Shells, basically really really big shotgun shells the tanks can use to "scratch each others' backs" if enemy infantry tries to swarm them. The tungsten balls fired by the shells can't do anything to the tanks' armor, but they'll do plenty to any squishy infantry trying to pry a tank hatch open.
 * The German J Pz 38(t) 'Hetzer' had just such a remote MG turret in 1944. Many other German tanks mounted a Nahverteidigungswaffe grenade projector for close defense.
 * The famous German design for a gun that shot round corners was actually used this way - a machine-gun firing upwards from inside the tank through the ninety degrees bent barrel could be swivelled through 360 degrees to sweep a tank exterior free of life's little obstructions. The bullet would fragment as it passed through the curve, making it even more effective.
 * Designs for the M60A2 had such a cupola turret as well.
 * In keeping with the desperate circumstances at the time, the Japanese attempted to extrapolate their successful Kamikaze program onto the battlefield to combat American tanks. Specifically, having Japanese soldiers effectively suicide-bomb American vehicles. One famous example featured a Japanese soldier charge an American tank, who then suffered the humiliation of watching the tank drive away from him faster than he could run. The tank traversed its turret...
 * This tactic was also used, with a similar rate of success, by Iran during the Iran-Iraq War in the 1980's.
 * The orginal Japnnese tactic was to have ten men attack at the same time, most would be killed in the attempt.
 * One (ageing) variant of an infantry-portable, disposable Light Antitank Weapon is meant to be fired at the engines of enemy armor, whereupon it ignites a thermal charge that's meant to burn out the engine. The basic mechanism works a bit like a high-tech RPG variant - however, as seen in a briefing video by this troper, this weapon has also proven to be extremely effective at cooking its crew alive if it managed to penetrate the crew cabin instead.
 * Witnessed for real during a NATO field exercise in Denmark: a particularly pissed-off British Para managed to lob a thunderflash (pyrotechnic designed to simulate a grenade) right into the open hatch of a German Army APC with a full complement of uncharacteristically dozy Panzergrenadiers inside. In the confined space, it ruptured the eardrums of thirteen German soldiers. The Bundeswehr was not amused and compensation was paid. The errant Para was officially bollocked, but privately congratulated by his Colonel for the accuaracy of his aim and for teaching the Jerries a good lesson - never drive in a combat zone with the lid open.
 * There is a story that the Russians trained dogs to associate the underside of tanks with food and then strapped mines to their backs, in an attempt to slow down the German advance. Too bad the dogs only associated the undersides of Russian tanks with food...
 * There is another story about how La Résistance in France managed to steal a German tank just by chucking a potato down the hatch. The crew thought it was a grenade and ran away, leaving the resistance fighters to assume command of it.
 * In Korean War, a company of British Centurion tanks got swarmed by Chinese soldiers climbing onto the tanks, attempting to do this en masse. The British Response? Turned their turrets on each other and sprayed the Chinese with their coaxialy mounted machine guns.