Riddle: You are standing at one end of a hallway with three light switches. At the other end there is a door with a light bulb behind it. You can only visit the room once and the door must be closed while you are at the end with the light switches. How do you figure out which light switch turns on the light?
Answer: You turn on the first light switch and leave it on for a while. You turn it off, turn the second one on, and go to the room. If the light is off but warm it is the first switch because lights get hot. If it is on it is the second and if it is neither warm or on it is the third.