A man is standing in front of three light switches and then there is a shut door. He is only allowed to go through the door once and there he will see three light bulbs. By turning any light bulbs on or off the man must be able to know which light switch goes with which light bulb. E.g. He can turn two on and know which switch is for the unlit bulb but will remain uncertain about the other two.
He turns one on, leaves it for ten minutes and then turns it off. Turn another on and walk in. One light bulb will be on, one off and one off and hot!