As the kids head off to summer camp, many of them will partake in the timeless tradition of trying to get their friends to pee themselves by slipping their hand into some warm water. Does this actually work, or have generations of campers wasted their efforts?
We can’t say for sure. There’s plenty of anecdotal evidence that suggests the prank works reliably, but, as they say, the plural of anecdote is not data. Some friend of a friend who swears this happened to him at camp decades ago doesn’t really count for much. To test things out in a controlled environment, the MythBusters once tried the trick on each other and a crew member in a lab with sleep-monitoring equipment and moisture alarms in the beds. Their results were less than impressive: zero wet beds (to be fair, though, a sample size of three isn’t great).
I’m not aware of any other scientific testing of the trick, and am not convinced that it does work, or that it doesn’t. It’s plausible, but light on evidence.
If you’ll join me in the Speculation Zone, though, I’d hazard a guess that if the trick does work, it would rely on the power of suggestion. We’ve talked before about how having to pee when you hear the sound of running water is a kind of conditioned response, spurred by the unconscious connection we make between the sound and the act. I don’t think it's out of the question that a similar connection could be at work here, but it doesn’t seem as direct. Wet hands are associated with urination through hand washing, which you do after relieving yourself, and there don’t seem to be many issues with people losing control of their bladders other times they get their hands in some warm water.
There is such a thing called “immersion diuresis,” which is urination brought on by temperature and pressure changes from immersing the body in water, but research suggests that whole limbs or the whole body needs to go under water for this mechanism to work, and a single hand isn’t enough.