Not so much a riddle as a... well, I'm sure there's a good name for it.
It could be used in any modern or future game, as well as any fantasy game where magic is used to simulate light switches.
You are in a room with only one exit. The exit leads downs a winding path to a second room. In your room there are three light switches, all initially set to Off. One, and only one, turns on a light bulb in the second room. Due to the winding path, you cannot see the light of this bulb from the first room. Once you exit the first room, the door closes behind you, and you cannot re-enter. Your task is to determine with 100% certainty which light switch controls the bulb in the second room. How do you do it?