The Country Road

You are standing in front of a closed room. Inside this room, there are three light bulbs. Outside the room, you have three switches that control these three bulbs.

The riddle: You can flip the switches as many times as you like, but you can only open the door once. Once the door is open, you must be able to tell which switch controls which bulb.

How can you do it?

Reflection

At first glance, the problem seems impossible. With only one visit, you can only turn on one bulb and see which one lights up. But there’s a trick…

The Solution

1. Turn on the first switch and leave it on for a few minutes.
2. Turn off the first switch and turn on the second switch.
3. Leave the third switch off.
4. Open the door.

You can now determine:

• The bulb that is on is controlled by the second switch.
• The bulb that is off but warm is controlled by the first switch.
• The bulb that is off and cold is controlled by the third switch.

Lessons

This puzzle perfectly illustrates the importance of thinking outside the obvious constraints. Temperature is a hidden piece of information that was not explicitly mentioned in the problem, but is perfectly usable.

“The most elegant solution is often the one that uses information everyone overlooks.”