An old-fashioned light bulb hangs from the ceiling of an otherwise empty room. A door leads outside, where three swithces sit on the wall. One of the switches control the light bulb - how do you figure out which switch it is?
You cannot see from outside the room if the light bulb is on or off, and you are only allowed to enter the room once. The light bulb and all the switches are initially in their off position. You are allowed to flip the switches any number of times, and you can take as long as you want before answering.
Use your common sense ;)