You are in a house. There are 3 light switches. Inside of a closed room (that you CAN NOT see) there is a light bulb that is turned on by one of the switches. If you can only go inside the room one time how can you tell which switch turns on the light?
You turn 1 switch on for ten min. Then you turn that one off and turn on the second one and go inside the room. If the light is off but the bulb is HOT it's the first switch. If the light is on it's the second switch. And if the light is off and the bulb is cold it's ther third switch.