This is the solution to problem 14 on the hard interview questions node. If you have not read the question, the following will make no sense to you:

Turn light switch 1 to the ON position for a few minutes, then turn it off. Then turn on switch 2, and go upstairs. The light bulb that is off and hot is attached to switch 1. The one that is on is attached to switch 2, and the one that is off and cold is attached to switch 3.