light-bulb-glowing-filament-light-blue-uncropped-lores-3-ahdTime for another classic.  Here we go…

There’s a standard filament-type light bulb in a closed room upstairs. It is controlled by one of three standard on-off switches downstairs. You are downstairs. How can you discover for certain which switch controls the light by operating one or more switches and then taking only one trip upstairs into the closed room?

The answer to this puzzle, and 100 others,  can be found in a new kindle ebook called PUZZLED, and is available in the UK here and USA here.

116 comments

  1. Like ButMadNNW, and horuskol I have AN answer. Took about 1 minute, but I am not convinced it is the correct solution!

  2. I knew the ‘trick’ to this straight away as I’ve seen it before-but I think it only works if the state of the light (on/off) is known at the start, which isn’t included in the puzzle statement so I might have misremembered it, or that assumption is implicit.

  3. Using other different-yet-related puzzles I’d heard in the past I reckon I got a definite answer in about 10 seconds.

  4. I have an answer but, as Geoff Coupe and Nick Sharratt said, it depends on one assumption which is not stated in puzzle’s text.

  5. yeah~I can finally solve the Friday puzzle in a few seconds! haha! It is because I have heard a similar puzzle when I was small. That question was that there were 3 light bulbs in a closed room upstairs and 3 switches downstairs. All other conditions are the same, only one trip is allowed.
    I think one light bulb is easier to solve 😛
    haha~

  6. Figured it out in about 30 seconds – was about to give up, but then it hit me. Googled to confirm my answer, and found that although it was basically the same thing, the Googled version I found was slightly less neat than my own – perhaps because I’m used to thinking in binary, which is what made this problem tricky – it’s *not* binary.

  7. I have an answer, but the kicker is that in the time that is required for my answer to have effect, I’m sure that an additional trip upstairs would be more practical!

  8. *Smashes controls, stomps upstairs, smashes lightbulb.
    ‘Hulk no like easy puzzles..!’

    Maybe Hulk needs to be more sensitive… 😉

  9. Like others here, I am assuming that the light is off at the start of the puzzle. That said, I am confident with my answer, and it took maybe a minute to nut it out.

  10. If there are several containers of water, varying in size, something to make a hole in each, and other beams and devices available, then I could do it…..

  11. Heard of this one before but couldn’t remember the answer. Knowing the sort of way that I needed to think helped though, got it in about 15 – 20 seconds. Again my answer assumes that all switches are off at the start.

  12. Why would you google the answer?

    And then why would you tell the world that you did so?

    You’re missing the whole point. Get your brain lubricated!

  13. Maybe one minute, involves a little bit of lateral thinking, but I think I got it right. Or at least I know my solution is one that works. 🙂

    1. I couldn’t wait so I looked up the answer! Where the puzzle is written elsewhere, a rule is specified which would prevent my solution. The real solution is elegant and practical. Good one.

  14. I spent a minute or two thinking it over. I have AN answer, but whether or not it is the one Dr. Wiseman is looking for is another question altogether. Suffice to say, my answer satisfies me, and I believe it would work.

  15. I know this puzzle and one (the only?) solution.
    Hm, as I remember, it’s impossible to discover the functional switch for certain WHILE you are still downstairs. You can be sure only AFTER going upstairs for the first time, can’t you?

  16. Took me about 5 minutes. This really is a thinking outside the box puzzle. Thinking back to the question, I should have realised the wording was important…

  17. For those who found a solution in seconds:
    In a bathroom a bathtub is filed with 10 liters of water.
    You have a spoon, a empty glass and 10 empty 1 liter bottles.
    What’s the easiest way to empty the tub?

  18. 5 minutes for an answer (but only because I found a clue in the way you asked the question – listening/reading crefully can be just as important as logical thinking, because without it I wouldn’t have cracked it)

  19. Remembered this puzzle from someone giving it to me a couple of years back. Took a couple of seconds to pull back the details. I do like that one.

  20. Not enough information provided. I have a quick easy solution, but i’m not sure if it is cheating or just thinking outside the box. Anyway, the way the question is worded does not prevent my answer, so it’s right, whether it is the intended answer or not.

    Actually, I have two answers, the second one is to get an amp meter. . .

  21. Richard left some assumptions off of the puzzle. I will berate him on Monday when he posts the answer.

    That said, I remember this one from my youth, so it only took a couple seconds to remember the answer.

    -Matt

  22. Wasn’t in a thinking mood when I first saw this. Spent a little time away. Came back, gave it three read throughs to get my mind working the problem and the penny dropped.

  23. Didn’t think it could be done.

    After 2 mins had an idea…might have been in the back of my mind from being a youngster…pretty sure it’s right.

    Feel VERY relieved.

  24. Just thought about it long enough to figure out another way. A few seconds really. Probably this one is the “right” answer.

  25. Hi,
    Well as technology is going very fast and in every electronic apparatus there are instruction which are in different languages by which we can easily understand language select to whom we know and will communicate in that language.

  26. Got something after I slept on it. Knew the general answer for 2 switches and 1 bulb, just needed to think about it. Only Monday will tell!

  27. Done. It took less than a minute. I thought it was a trick question before remembering all the mystery lights and switches I’ve had to play with in my life.

  28. I have AN answer, but it hinges more on the properties of the bulb than it does on any sort of mathematics. Curious to see if it’s the correct one.

  29. it took me all the time while reading through all the comments above.

    Pretty sure of the answer I have got. No extra assumptions needed, of course.

  30. got it. Once I actually thought about it instead of saying “hrm. not sure. moving on to my email!” it didn’t take long. maybe a minute or two. but like everyone has said, I have an answer, a practical answer, don’t know if it’s the one you’re after.

  31. Didn’t give the quiz a lot of thought. I was amazed that the answer after only typing in 4 words was on the front page of google. How is worthless info like this ranking at the top of google. I sell tickets on line and spend hours on SEO for my website. http://www.LuckyDayTickets.com I have an honest business and save people money all the time if they can find me. I sell Premium Concert and Sporting event tickets with no service fees, with a very competitive low markup. I wish I could rank like this under Concert tickets. I have a quiz. How do I acomplish 1st page serach ranking under concert tickets with out spending a fortune in advertising. Lets see how many if any correct answers I get

  32. When I heard it for the first time I couldn’t come up with a solution and I gave up. About 2 hours later I had an epiphany.

  33. I’ve never heard this or anything like it, and it took me about 30 seconds. As I scrolled down here and read other posts, I realized I was assuming, like a lot of others, that the bulb was initially off. However, the fix for this is pretty obvious, and came to me almost instantaneously.

    The key to the solution is in the statement of the problem. Once you read the proposition a second time you realize the odd emphasis that gives away the answer.

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