There is a room, with no windows and a door, which can be lit by a light bulb hanging from the ceiling. The door and walls are opaque and solid, and when the door is shut there is no way to tell if the light inside the room is on or off from outside.
There are 3 switches (lets call them A, B and C) on the wall outside the room - only one of them operates the light, the other two do nothing. All the switches are off to start with.
You are outside the room, the door is shut. How will you find out which switch operates the light, if you can only open the door once, and once it's open you can't touch the switches?