brain teaser

A single light bulb is in an enclosed room. The room has a single door and no windows. The light bulb is controlled by one of three switches. The switches are outside the room. The other two switches don’t control any other light bulbs (or anything else).

How do you determine which switch turns on the bulb if you can enter the room only once and after you open the door, you can’t touch the switches again?

The first reader to answer correctly will win… a… prize.

Of some sort.


No cheating!

This brain teaser was shamelessly stolen from

  1. Holly

    can you just turn all the switches on and enter the room?

  2. Nils and/or Katrina

    you lost me at light bulb.

  3. Liz

    well, if the doors are like ours at home then you just look for the light that shines from under the door while trying the switches.

  4. Dave

    Holly: Nope. How would you know which switch is the one that is controlling it?

    Liz: I forgot to mention that the door is an air-tight, crackless door.

  5. Melissa

    Take apart the light switch and see which one is connected to a wire.

  6. Dave

    Melissa: It’s an impenetrable light switch cover. Sorry.

  7. Holly

    But don’t you see Dave, by turning on all three switches the light is on no matter what.

    I’m just trying to take the easy way out here.

  8. Dave

    Holly, you are making me sigh right now…

  9. Joe

    I was asked that question once in an interview and completely lucked into the answer. I didn’t get the job though…

    Turn on switch #1, wait a few minutes, then turn on switch #2 and enter the room. If the bulb is on, it’s switch #2. If the bulb is off and warm, it’s switch #1. If the bulb is off and cold, it’s switch #3.

  10. david

    wow- impressive. I was just about to say what joe said (whoever that is–sorry joe, I don’t think we’ve met) but he beat me to it. funny how that happens. My question in my interview was about elephants in the fridge. You know, how to get one in–morbidly I answered: “Blend it?” Sick I know. But in my case I got the job. Keep in mind this was for a $6 an hour job on the BYUH campus. 6 bucks! And my interview was a psychosis analysis for over an hour. More where that came from…another time.

  11. david

    I think I made up the phrase psychosis analysis– it sounded good in my head.

  12. Dave

    Nicely done, Joe. That is the correct answer. You win… a prize… of some sort.

  13. Joe

    What is the real answer to the elephant question? The only similar joke I’ve heard is:

    “how do you get a BYU coed in the fridge?”

    “grease her hips and throw in a twinkie”

    I didn’t make it up–don’t kill the messenger.

  14. Beth

    Ok, here’s one at my level – How do you get an elephant into a Safeway Bag? Hint: Take the “S” out of “Safe” and the “F” out of “Way”. (Katrina, I liked your line about being lost at “lightbulb”.)

  15. Dave

    But there is no “F” in way.

  16. Beth

    heh heh heh. What a naughty joke. (Thanks for playing.)

  17. Holly

    beth, i love the sesame street laugh.

  18. Joe

    By the way, that was only my second favorite BYU co-ed joke. My very favorite goes like this:

    Q: How do you know a BYU co-ed is walking behind you?

    A: You can hear her knuckles dragging on the ground.

    P.S. I love BYU co-eds.

  19. Peter

    I thought about this for a minute. Then realized these are always simpler than they sound. So, can you do it without even going into the room, I asked? Sure. Just open the door (and leave it open, presumably).


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