You are standing in a hallway with three light switches. Each switch has an accurately labeled on and off position. In an adjoining room, behind a closed door, there is a bare lightbulb, which is controlled by one of the three switches. The other two switches are not connected to anything. You are free to manipulate the switches any way you like and for as long as you like, and you are permitted to enter the room containing the lightbulb once and only once. If and when you exit the room, you must close the door behind you – you are never permitted to view the lightbulb while manipulating the switches.
Here’s your challenge: figure out which of the three switches controls the lightbulb.
Solution: This blog may not have a lot of visitors but I sure do have some smart, creative readers. The comments below make one thing abundantly clear: there are multiple correct answers to this week’s puzzle. People found solutions involving roasting human hair, monitoring power meters, breaking the switches, installing a doggie door, self-electrocution, and several other ideas, which you can read about yourself below. My favorite solution, and the one I had in mind, goes as follows:
- Set switch #1 to the on position and switches #2 and #3 to the off position.
- Wait ten minutes.
- Turn switch #1 off and switch #2 on.
- Enter the room and examine the bulb. If the bulb is on, then it’s controlled by switch #2. If the bulb is off and warm or hot, then it’s controlled by switch #1 (because it was recently left on for ten minutes which caused it heat up). If the bulb is off and room temperature, then, by process of elimination, it must be controlled by switch #3.