How does this prank work?

Maybe GQ, maybe not. I’m never sure.

Everyone’s done it, had it done to them, or heard of someone who did it/had it done to them…the old Put the Hand of Whoever Falls Asleep First in Warm Water. Ivylad says it never fails.

What is the physical mechanism that makes one pee when their hand is in warm water while they are asleep? And how come the need to pee does not wake you up first?

Apparently it’s just the mental connection between running water and urination…

Or at least that’s what Cecil Sez…