The Switching Rooms
Three light switches are outside a closed door. Inside the room are three light bulbs, each controlled by one switch. You can only enter the room once. How can you determine which switch controls which bulb?
Turn on one switch for 10 minutes, then turn it off. Turn on a second switch immediately. Enter the room - the bulb that is on is controlled by the second switch, the bulb that is warm (but off) is controlled by the first switch, and the cool (off) bulb is controlled by the third switch.