Ten prisoners are summoned before their king. They are told that they will participate in following riddle, deciding whether they are worthy to be freed, or executed. The prisoners are allowed to talk one evening, to discuss a strategy. After that, they will be put into separate cells. Once an hour, one random prisoner will be led into a room, containing two switches. The  switches can be “on” or “off”. In the beginning, both are “off”. The prisoner has to change one switch position, but is not allowed to change more than one. After that, he is brought back into his cell. The quest for the prisoners is to know when all of them have been into the switch room at least once. They do not have to know the exact time point... but to be freed a prisoner has to call the king and tell him that now all prisoners have visited the room at least once.