The 3 Bulbs & 3 Switches
The Puzzle: You are in a room with 3 switches. In the next room (which you cannot see), there are 3 light bulbs. You can only enter the room with the bulbs once. How do you find out which switch controls which bulb?
1. The Lateral Thinking
Most people think about “Light” as the only output. But bulbs also produce Heat.
2. The Solution
- Turn on Switch 1 and wait for 5-10 minutes.
- Turn off Switch 1 and turn on Switch 2.
- Enter the room immediately.
3. The Mapping
- The bulb that is ON: Controlled by Switch 2.
- The bulb that is OFF but HOT: Controlled by Switch 1.
- The bulb that is OFF and COLD: Controlled by Switch 3.
Interview-Focused Questions
Q: What if the bulbs are modern LEDs that don’t get hot?
A: This is a great follow-up. In that case, the traditional “heat” solution fails. You would have to look for other physical properties or hope the switches are in the same room as the door (allowing you to see the reflection/gap under the door).
Q: Could you solve this for 4 switches?
A: With only one entry and only using Heat/Light, you can only distinguish 3 states (On, Off-Hot, Off-Cold). To solve for 4, you would need a new variable (like a dimmer or a specific sequence) or multiple entries.
Key Takeaway
This puzzle tests Lateral Thinking. It requires you to look beyond the obvious binary state (On/Off) and consider secondary effects (Heat).
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