In your cellar there are three light switches in the OFF position. Each switch controls 1 of 3 light bulbs on the floor above. You may move any of the switches but you may only go upstairs to inspect the bulbs one time. How can you find the switch for each bulb with one inspection??

Number the switches 1, 2 and 3.

Switch on number 1 for 1 minute, then switch it off. Switch on number 2. Go upstairs…
The light that is on is connected to switch 2. The light that is off and warm is connected to switch 1. The light that is off and cold is connected to switch 3!!