A classic room puzzle with three switches and one light. Find the method that always works. From the free puzzle set on Think Fanny. Next try Fox, goose, and grain.
The puzzle
You are outside a closed room with three switches. Exactly one switch controls a single light bulb inside the room. The other two do nothing.
You may flip the switches as much as you like before you open the door, but once you enter the room you cannot go back out to try again.
How do you figure out which switch controls the bulb?
Answer
Turn switch 1 on for a few minutes, then off. Turn switch 2 on. Enter the room.
If the bulb is on, it is switch 2. If the bulb is off but warm, it is switch 1. If the bulb is off and cold, it is switch 3.
This answer was re-solved from a cold start before the page went live. If you can break it, tell us and we will fix it.
Why the warm-bulb step works, and when it does not
The method leans on a filament getting hot, which is worth knowing before you try it on a modern fitting that barely warms up at all:
