The question
There are three switches outside a room. Only one lights a bulb inside. You may flip switches freely, but you can enter the room only once. How do you identify the correct switch?
Logic lab
Light Switches
One visit, three switches, and one clever trick keeps you from guessing blindly.
Hints
Use both sight and touch—brightness alone is not enough.
Answer
Turn on the first switch and wait a minute, then turn it off and turn on the second switch. Enter the room once. If the bulb is lit, the second switch is correct. If it is dark but the bulb is warm, the first switch was correct. If it is dark and cool, the third switch controls the light.