I think it's along the lines of "you're in a room with three lightswitches. They control three lightbulbs in a different room (so you can't see them from the room with the switches). You get some time to use the switches, and then you go to the other room and have to guess what switch controls what lightbulb. You aren't allowed to go back and flip the switches again once you leave." The solution generally is to flip one switch, leave one off, and flip the last one on for awhile but then turn it off just before you leave to go to the other room. The lightbulb that's lit obviously goes with the switch you flipped on, and the other two are off. One of these two will be warm though, because it was recently turned on, and that one goes to the switch you flipped on and then off.