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
Wry extract from a pretty useless forum thread on The Naked Science Foundation:
Wikipedia says (but the cite is a dead link):
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