You're in a room with three light switches. Each controls one of three light bulbs in another room. You can only enter the other room once. How do you determine which switch controls which bulb?

You're in a room with three light switches. Each controls one of three light bulbs in another room. You can only enter the other room once. How do you determine which switch controls which bulb?

Approach: Use heating property of bulbs.

Turn switch 1 on for 10 minutes, then turn it off. Turn switch 2 on. Enter room: The bulb that is on is controlled by switch 2. Touch remaining bulbs: The warm bulb is controlled by switch 1, the cold by switch 3.

Follow-up: What if bulbs are LED and don't heat up?

Key concepts: heating, time delay, observation.