3 Bulbs and 3 Switches

This puzzle is perhaps not as ‘pure’ as the others, it doesn’t reduce to a mathematical model. But it is quite a common question asked in interviews so it’s worth looking at it…


You are in a room with 3 switches which correspond to 3 bulbs in another room and you don’t know which switch corresponds to which bulb. You can only enter to the room with the bulbs and back once. You can NOT use any external equipment (power supplies, resistors, etc.). How do you find out which bulb corresponds to which switch?

3 bulbs and 3 switches puzzle


Before reading the answer if you are interested in a clue?

Clue: Light Bulbs ¬†get hot when they’re on.

Clue didn’t help you ;)

Here is the solution for you.

Switch on switches 1 & 2, wait a minute and switch off number 2.

Enter the room. Whichever bulb is on is wired to switch 1, whichever is off and hot is wired to switch number 2, and the third is wired to switch 3.

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