Three Lightbulbs

In the comments a couple entries ago, someone mentioned a lightbulb-centric riddle I solved during one of the interviews I did for my current job. Many of you have probably already heard it, but for those that haven’t heard this one yet, it does make you feel good when you figure it out. In that spirit, the riddle:

There are two windowless rooms, connected to each other by a door. In one room are three lightswitches, and in the other room are three lightbulbs. Each switch controls one of the three lightbulbs. You are in the room with the lightswitches, and the door is closed. You are allowed to open the door after manipulating the switches and go into the lightbulbs room, but you are not allowed to re-enter the lightswitches room once you leave it.

Which switch controls each bulb?

Rules and suggestions while wrestling with this one:

  • If you already know the answer, for Pete’s sake, don’t post it in the comments!
  • If you’re trying to figure out the answer, it’s probably best to not read the comments.
  • If you figure it out and leave a comment, please type something obvious in ALL CAPS letting people know you’re about to answer it so they don’t read your comment if they don’t want to know the answer yet.
  • You have no additional supplies with you in the rooms. The answer isn’t a dumb “gotcha” kind of answer like, “See light under the door, blah blah.” It’s straightforward. I’m not sure if my riddlers told me that when they asked this of me, but I think it makes the riddle less frustrating, while still allowing it to be challenging.

Go for it!

9 thoughts on “Three Lightbulbs

  1. Not too obvious but could spark ideas…
    Reminds me of the google job/interview ads. Sounds like they’re just looking for people who think about the effects what they do actually has on things than just what they immediately see.

  2. three light bulbs and three switches? are these osha approved light bulbs and switches or are we talking extension cords running across the floor? can i rewire the switches? can i use colored bulbs instead? i’m pretty sure i have it figured out, but it’s more fun to post misleading questions than a solution. also… check your email for ‘how to dismantle a pop song’

  3. OK, I think it’s time for those who have solved it to step forward and give their answers.

    If you had already heard the riddle elsewhere, step back and let someone else try for the answer. If you read it for the first time here, go for it!

  4. I believe that if they’re flourescent bulbs you’re in trouble.

    A co-worker mentioned the problem once-as a difficult one–and I came up with the answer more quickly than he would have liked.


    Turn one switch on for five minutes, then flip it off and flip one other switch on. Run into the next room and you will find the first switch you switched belongs to the unlit bulb which it hottest to the touch, the cool bulb belongs to the swtich you didn’t flip, and the only bulb that’s on – well, I don’t think this needs any more explanation.

    Is this right? Is there another way?

  6. I’m not aware of any other method than that basic idea. Good work, Dave! 🙂 The key is to realize that lightbulbs have more varying characteristics than just on/off. They get hot and cool down, too, and probably other stuff I can’t think of.

  7. Woot! I knew I was Apple material! Although that Apple’s hiring criteria can be met by a university freshman at 6:00 AM doesn’t hold much promise for the company’s future…

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