When I was in high school, we tried the hand-in-warm-water thing on my friend's sleeping little brother. We couldn't tell if he wet the bed at the time (was the middle of winter, tons of blankets) but the next day, he was up before all of us, doing laundry. We were going to give him a hard time about it, but the look on his face was so horrible, we actually let it go.
And for the record, we weren't just being mean bitches. My friend wanted to get him back for selling her a bag of oregano. Even though she was stupid enough to fall for it, siblings can't let these things go unpunished.