You are standing in a hallway next to 3 light switches that have been switched off. There is another room down the hallway with 3 light bulbs – all are incandescent light bulbs and each light bulb is operated by one of the switches in the hallway.
The door to the room is closed and you cannot see the light bulbs from the hallway.
How would you figure out which switch operates which light bulb, if you can enter the room with the light bulbs only once?
Let’s name the switches S1, S2 and S3.
Turn on switch S1. After 5 minutes, turn off switch S1 and turn on switch S2. Switch S3 is never turned on.
Immediately enter the room. The light bulb that is on, is controlled by switch S2. Check the remaining 2 light bulbs, the one that is hot is controlled by switch S1. The remaining light bulb is controlled by switch S3.