It's an old trick often done by The Prankster. You put someone's hand in warm water while they are asleep and they pee. Simple as that.
This may or may not be Truth in Television. There is such a thing called Immersion Diuresis, which is urination brought on by temperature and pressure changes from immersing the body in water. However, research suggests that whole limbs or the whole body needs to go under water for this mechanism to work, and a single hand isn't enough. The prank was actually tested by Mythbusters, but they didn't find any evidence that it works. However, very rarely for very few people, it may possibly work.
- In Diary of a Wimpy Kid, one of the books has Greg in a sleepover with Rowley and a bunch of younger (around six to eight years old) boys. Some of the boys sneak up on Greg with a bowl of warm water, but he's actually awake so he runs away.
- Discussed in one episode of A.N.T. Farm where Chyna makes Cameron believe he's a medium. After Cameron "predicts" Angus will have an accident, Sierra suggests putting his hand in warm water, to which Chyna responds that they're not going to give Angus that kind of accident.
- In one episode of House, House pulls this prank on his friend Wilson, who's crashing on his couch at the time. When House goes to sit on the couch at the end of the episode, Wilson advises him to move.
- M*A*S*H: Hawkeye and Trapper pull this bit on Frank Burns in an early episode of the series. Just one of the many pranks they've played on him.
- In Mystery Science Theater 3000, in the first host segment of "Zombie Nightmare", the Mads send the SOL crew a voodoo kit. Mike however uses it to make a Voodoo Doll of Dr. Forrester, and puts the dolls' hand in water, resulting in Dr. Forrester peeing his pants.
- In the Homestar Runner Halloween Toon "Halloween-Potion-Ma-Jig", Homestar is sent to collect ingredients for Marzipan's Halloween potion and has to get essence from Strong Bad, Strong Mad, and The Cheat. If he tells them he is trying to ruin her Halloween potion, them give him "Essence of Putting Strong Mad's hand in warm water while he's asleep".
Strong Mad: MY SECRET SHAME!
- American Dad! had an episode where Steve and his friends are throwing a slumber party. Flashbacks show them putting Barry's hand in warm water and making him pee himself. Another variation in the flashback shows them putting Toshi's hand in warm water, but Toshi doesn't pee himself, but Barry does instead.
- In the As Told by Ginger episode "Sleep On It", Miranda and Mipsy plan to put Ginger's hand in warm water during a sleepover at Courtney's house. It fails because Hoodsey, who was also there to steal the petrified eyeball back from Blake, overhears their plan, and takes the water from the sleeping Ginger. He then drinks it and promptly falls asleep, resulting in him wetting himself.
- Family Guy
Doctor: Well, I hope you're proud of yourselves! Fourteen premature births!
- There was one episode where Chris videotapes what happens when he is asleep, and Peter puts Chris' hand in a bowl of warm water to make him pee.
- In "The Thin White Line", Peter and Brian, who are staying at a rehab center, pull this prank on a room full of pregnant teenagers, causing all their waters to break.
- In The Loud House episode "Feast or Family", it's revealed that Luna and Luan have an agreement: the former is not allowed to play her gong at night, and the latter is not allowed to put the former's hand in warm water.
- In one South Park episode, Butters falls asleep and Cartman puts his hand in a cup of water, but gets what's supposed to happen next wrong and pees on him instead.
- In the Total Drama Island episode "The Big Sleep", Duncan pranks a sleeping Harold by putting his hand in a mug of warm water, causing him to pee his pants.
- In an episode of Robot Chicken, the Nerd is telling stories to his niece and nephew and falls asleep to dream about his stories, as he does. The kids decide to just pee on him, but he wakes up and thinks that they pranked him with the hand-in-the-water prank... before wondering where the cup of water is.