Brain Teasers · The Three Switches
Three Light Switches Puzzle - One Trip, Played
Three switches, three bulbs behind a closed door, and one look inside.
What is the three switches puzzle?
On and off gives two outcomes, and you have three switches to tell apart, so light alone can't do it.
Heat is a second signal: an incandescent bulb stays warm for a while after it is switched off.
How it works
Three switches in a corridor each control one of three bulbs behind a door. You can flip the switches as much as you like but go in only once. The story asks how to match them up. You pick one of four answers. Then you step through the answer and the breakdown.
How scoring works
This teaser checks your answer. A wrong pick is crossed out. You get two tries before the answer opens. A correct first try earns a ticket; a second try or opening the answer early is recorded but earns nothing. The story teasers share one lifetime unlock for free users.
The key insight
Turn switch 1 on for ten minutes, turn it off, turn switch 2 on, and go in. Lit is switch 2, dark and warm is switch 1, dark and cold is switch 3.
The full step-by-step solution lives at /brain-teasers/three-switches/solution.
A worked example
Inside, one bulb is lit. Touch the other two: the warm one was on for ten minutes (switch 1), the cold one never came on (switch 3).
Common mistakes
• Saying it can't be done because you only consider light.
• Leaving switch 1 on too briefly for its bulb to warm up.
• Not mentioning that LED bulbs would break the trick.
Why interviews test this
This tests counting the outcomes you need, then looking for a signal the puzzle didn't mention. The written solution is at /brain-teasers/three-switches/solution.
Play The Three Switches on Outcry
The Three Switches is free to play on Outcry, in the Brain Teasers section, and every run is timed and scored. Read the guide once, then play The Three Switches and check your score against the strategy above. The guide book has a written guide for every other game, and the practice hub and mock assessments put the same skills under a real screen's clock.