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?
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.