This is another spoken game often played at camps, or at least that's where I learned it. It's one of those where you try to figure out what the rule is as you play (like the Green Glass Door, but a different rule). (Looking it up or asking someone who does know is not allowed - and sure, if you do it we may never know, but you'll know and hopefully you'll feel guilty so just don't cheat ) Anyone who does know the rule, don't say it!
I'm going to take an ashspren on the trip.