An old chestnut goes like this:
There are three light switches on a panel. They each control a light in another room, but you can't see any of the lights from the location of the switches. All the switches are initially in the off position, and each one turns one of the lights on when it is flipped to the on position. How can you determine which switch controls which light while only making one trip between the switches and the lights?
Note: This puzzle makes an assumption about the lights that was once true, but is not necessarily true today. Saying what this assumption is gives away the answer, so I won't say here, but keep this in mind. (This is something other than the usual, that the lights all work and all the statements above are true.)