Riddle: You are outside a door to a room which you cannot see in. There are 3 light switches by the door. 2 dont work. how do you find which works for the light in the room if you can only touch 2 light switches before opening the room? Clue... Propities of light.
Answer: Turn on the first and wate 5min. turn it off and switch on the second. enter the room. If the bulb is hot, it is the first. If it is cold and off, it is the last. if it is on, it is the second