Brain Teasers
Hard
4 min
A man walks into a room containing three switches. Each switch controls one of three bulbs in another room. He can operate the switches as much as he wants, but he may enter the bulb room only once. How can he determine which switch controls which bulb?
Hint
Use heat as well as light.
Answer
Turn switch 1 on for several minutes, turn it off, turn switch 2 on, then enter the bulb room. The lit bulb belongs to switch 2, the warm unlit bulb belongs to switch 1, and the cold unlit bulb belongs to switch 3.
Explanation
The bulbs provide two observable properties: whether they are lit and whether they are warm. This creates three distinguishable states.
Step-by-Step Solution
1. Turn switch 1 on for several minutes.
2. Turn switch 1 off.
3. Turn switch 2 on.
4. Leave switch 3 off.
5. Enter the bulb room.
6. Lit bulb = switch 2.
7. Warm but unlit bulb = switch 1.
8. Cold and unlit bulb = switch 3.
2. Turn switch 1 off.
3. Turn switch 2 on.
4. Leave switch 3 off.
5. Enter the bulb room.
6. Lit bulb = switch 2.
7. Warm but unlit bulb = switch 1.
8. Cold and unlit bulb = switch 3.