a stage has 3 lights, left, centre and right. however the only 3 switches are in a back room and aren't labled.
your job is to find out which switch controls which light. (it is not necessary to lable them yourself just work out and know which is which)
however when you are in the backroom there is no way of telling (through reflections or shadows ect) which switch controls which light.
you start off in the backroom with non of the lights on and are allowed to flick the switches as many times as you like then go onto the stage to see the lights but after you have done so you cannot return to the backroom.
Turn one of the switches on for a minute, then turn it off & turn another one on.... go into the room, see which is on, feel the other two bulbs to see which one is hot from having been on... job done.