Overview
This classic uses a resource beginners forget a bulb has: heat. You can only see the bulb once, so a single yes/no observation isn't enough — but a bulb carries two independent signals.
How to solve Three Light Switches
- Turn switch A on for several minutes so its bulb warms up, then turn it off.
- Turn switch B on and immediately enter the room.
- Lit → B; off but warm → A; off and cold → C.
The key insight
A light bulb encodes two bits, not one: on/off and hot/cold. Two independent signals distinguish three cases in a single look.
Variations & echoes
- The same 'find a hidden second channel' idea powers many one-shot measurement puzzles.
- With more switches you can chain more states (brightness, timing) to identify them.
Frequently asked questions
What if the bulbs are LED and stay cool?
Then the heat trick fails and one visit is genuinely insufficient — the puzzle assumes an incandescent bulb.