This is like an urban myth and a classic prank, but I never saw it in action.
Supposedly if someone is passed out or sleeping, the contact with water will make him pee. Is this true? If true, why does this happen?
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OK, I'm gonna try a few links on you, not sure how "approved" they will be:
From The Straight Dope
The effectiveness of the stunt is a matter of debate. Some urologists scoff at the idea. But other medical types have been known to tell patients having a tough time urinating after rectal surgery to put their hands in warm water.
Wry extract from a pretty useless forum thread on The Naked Science Foundation:
Short answer no with a but..
long answer yes with an if...
Wikipedia says (but the cite is a dead link):
The mechanism is unclear. The phenomenon (...) of immersing the hand of a sleeping person in water to make this victim urinate in sleep, although the efficacy of the trick is disputed.