I came across this puzzle:
You and 5 unfortunates are [to be] locked up [somewhere]. You all know that there is a single toggle switch initially in the UP position[,] to be found in a special room. Prisoners are kept apart and cannot communicate once the [period of imprisonment] starts. Prisoners are taken from their solitary cells one by one and one at a time, in no [particular] order, to this control room.See if you can do it. Be honest. Don't try looking up the answer... anywhere (unless you're giving up). Encouragement: I got there - after being baffled by it for a while.
The only way out... is if one of you declares correctly that all of you have visited the room at least once. The downside is that you will [all] stay there [forever] if the statement when uttered is made incorrectly. You have a few moments together before the [period of imprisonment] starts[,] to set up a strategy. No one else may interfere with the room. How can you arrange for one of you to be in a position to know that 'we have all of us visited the room at least once'?