Riddles, brain puzzles and mathematical problems

[Bridge]

Light bulb

There is a room that contains one light bulb. Just outside the room there are three light switches, but only one of them is connected to the light bulb inside of the room (the other two don't do anything). You are allowed to set the switches in whatever position you like (they only have two states - on and off) and you can then enter the room. Note that you may only enter the room once (opening the door counts as entering it) and you cannot peak through the keyhole or any other opening. Find out which of the switches controls the light bulb.