Three Switches, One Bulb

Question

Three wall switches outside a closed room; exactly one controls the incandescent bulb inside (the others do nothing). You may flip switches as you like, but may open the door and enter once. How do you determine which switch controls the bulb?

Accepted Answer

Flip switch 1 on for ten minutes, then off. Flip switch 2 on and enter immediately. - Bulb lit → switch 2. - Bulb dark but warm → switch 1. - Bulb dark and cold → switch 3. The information framing: one visit observing only on/off yields 2 outcomes, not enough for 3 hypotheses. The puzzle is a hunt for a second physical channel, and heat gives a third distinguishable state (you can't beat 2 with light alone, so any correct answer must smuggle in extra state). This is the canonical "expand your observable space" teaser: the constraint ("enter once") is binding only under an artificially narrow model of what you can measure. Interviewers use it to watch whether you challenge the implicit model rather than brute-force within it, the trading analogue is realizing a price series isn't your only data when you also have volume, latency, or queue position.

Three Switches, One Bulb

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