Prison escape (**)
There are a bunch of prisoners on a bus planning a jailbreak. There is a bathroom on the bus and they can ask to go whenever they like. They need a way that one prisoner will be able to determine when all the other prisoners have been to the bathroom at least once. In the bus the prisoners are in complete isolation and will never ever see each other, so the only communication mechanism they have is that each of them can leave the toilet seat up or down (which might be based on whether they found it in the up position or the down position when they entered the bathroom). The guards will never change the toilet seat's position. The prisoners can all communicate freely and make a plan in the jail before they leave. It's only necessary that one of the prisoners eventually becomes certain that all of the prisoners have been to the bathroom at least once.
Note: This isn't a “trick riddle”. You actually can (and have) to solve it just using the way they set the toilet seat.