[sammyg2]

You find yourself at the foot of a flight of stairs.
Next to you are three switches.
One of them turns on an incadescent light bulb in a room at the top of the stairs, but you can't see the light or any light from it unless you climb to the top of the stairs.

You need to determine which of the three switchs turns on the light but you are only allowed to climb the stairs one time.

How do you do it?