Time 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.

1. I have AN answer. Whether it’s THE answer remains to be seen. Monday. 😉

2. Martin says:

Immediate – but I remembered how to do it.

3. fpilot says:

Less than 30 seconds to Google it. Otherwise I would have sworn it couldn’t be done.

4. Victor Meldrew says:

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

5. I have an answer, it took me.. hmm. 5 seconds or so. It would work.. not sure if it is THE answer 🙂

6. I have an answer – took a couple of minutes – but it depends on certain assumptions holding true.

7. Right away, but I’ve run across this one before, and remember it from the last time.

8. hp88 says:

Heading to Google to see if my answer is right. 15 secs. It can’t be right, can it?

9. hp88 says:

ok, so it wasn’t step outside and look up at window. Hangs head in shame.

10. I have an answer I feel pretty good about. It took me about 1 or 2 minutes

11. Thought about it for a minute or so, but was too curious, so I googled it. I would have thought of that!

12. Nick Sharratt says:

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.

13. thirdman says:

I have one but it assumes the bulb is off to start with and all three switches are in the same position

14. @brianhalliday says:

Easy, but only because I’d seen it on one of Derren Brown’s shows.

15. Raja says:

I have an answer too. It took me about 3 minutes. I think it is THE answer but we’ll see on Monday 🙂

16. pete says:

It took me a couple of minutes to come up with an answer but I’m not sure if it is the right answer

17. thethreelimes says:

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

18. Cosmin Romania says:

Too simple. 5 secs.

19. I’ve got an answer that works for sure; no idea if it’s THE answer. It’s good though, so I reckon it is :]

20. Took about 15 seconds, but I think I’ve heard it or something similar before.

21. Pribi says:

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.

22. mangochan says:

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~

23. 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.

24. stunt_girl says:

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!

25. stunt_girl says:

26. Hulk says:

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

Maybe Hulk needs to be more sensitive… 😉

27. 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.

28. Gib says:

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…..

29. Helen says:

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.

30. Veljko says:

Got an answer. Took me about a minute or so, but I knew of the type of puzzle in question which helped.

31. david dredge says:

2 minutes, and without cheating!

Great question

Just found this blog – its brilliant

32. David Dredge says:

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

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

33. Alan says:

Got it. Pretty easy.

34. Captain says:

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. 🙂

35. An easy one for me 🙂
Engineers should all know this one!

36. I have a lateral solution (about a minute), don’t know if there’s a more logical one.

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.

37. Benjamin says:

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.

38. Unfortunately I think I remember a kind of inversion of this puzzle – but I stand by my ‘five seconds’ answer proudly!

39. Engywuck says:

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?

40. 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…

41. Andi says:

42. Engywuck says:

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?

1. Frostie says:

Pull the plug 🙂

43. J says:

@Engywuck – Remove the tub drain stopper.

5 secs

44. Rebecca says:

I’ve heard this one before, so I knew the answer already, but I do think this is a great puzzle.

45. Jeff says:

Got it immediately 🙂 I like this puzzle.

46. I knew it right away, but I’ve heard it before. If I hadn’t, I don’t think I’d ever have figured it out.

47. Stephen James says:

I think I got this one, about 20 seconds.

48. FrankN.Stein says:

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)

49. Frostie says:

Remembered the puzzle from some years ago but it took me a couple of minutes to remember how to solve it 🙂

50. SeeSome says:

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.

51. Gus Snarp says:

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. . .

52. Matt says:

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

53. livedman says:

Less than 3 seconds as Derren Brown did this puzzle on one of his shows.

54. Vern says:

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.

55. John Pilkington says:

5 hours. Richard really ought to rename his book.

56. Steffen says:

took me about one minute to get AN answer. Dunno if it’s correct though.

57. dannyness says:

58. Liz says:

10 seconds, this is a fun one!

59. I recognised this straight away, it’s one of my favourites.

First time I heard it I got it pretty quickly though.

60. What kind of a silly set up is this? Call the electrician and get the house rewired! Reminds me of when the bathroom light switch is outside the bathroom door. I hate that.

61. Ali says:

About 30 seconds. I like this one, though I think I’ve heard it before. 🙂

62. noamgr says:

Around 5 seconds, but I have a vague recollection of hearing a similar puzzle when I was a kid, which pushed me in the right direction.

63. Marktech says:

I got this immediately: I often use this sort of McGuffin at work.

64. Marktech says:

I got this immediately; I use just this sort of McGuffin at work sometimes.

65. Steve Bent says:

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.

66. slightly skeptical says:

67. Jason says:

I’ve heard this one before. Easy enough if you think about light bulbs.

68. I came up with a way to try it in under a minute, but I’d have to see if it’d work in the real world.

69. took me a while. And yep. I know the answer…
The force is currently with me. Muahahaha!!!

70. Sometimes, you can hear the circuit start running on the switch if you listen closely….

71. Gus Snarp says:

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

72. Gene Johannsen says:

Took me a couple minutes but I’m pretty sure I got it.

73. 10 min to solution. Was not conventional though.

74. Alan says:

Immediate, because I’m always asking this one too. But as 3 bulbs with 3 switches.

75. 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.

76. marie says:

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!

77. brainiax says:

thats not wise at all.

78. golden spirit says:

with the help of my girl friend 20sec 🙂

79. Vikram says:

about 45 seconds.. but feels good to have solved it!

80. 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.

81. Mchl says:

Alas, I knew this puzzle before. I like the solution though.

82. raikun07 says:

hmm.. good luck to those who tried to answer. 😀

83. Berber Anna says:

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.

84. I have a answer, I think. I will be interested to see if it is right though

85. Bu says:

86. Hai Pham says:

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.

87. Avery says:

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.

88. Michael K Gray says:

89. thought i had it while reading but then i realized i was wrong lol

90. jan says:

got it, it is easy, however it took me four minutes

91. 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

92. I didn’t give this quiz much thought. Answer coming on Monday.

93. k1point618 says:

pretty fast, 30 seconds?

Touch the bulb!

or

Put a mirror so it reflects where you can see the light come on.

96. ldmartin1959 says:

I think I got it. At least, it makes sense to me. Less than a minute.

97. Bob says:

Easy, got it in 3 seconds. Are the puzzles this easy?
Bob

98. Ingrid says:

Ok, I cheated, but now I got it. That nut was a good one!

99. boris says:

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.

100. Hi! You could just tell another person to go upstairs while you flick the switches and have them report to you their observations 😀 Don’t you think so?

~Double Mvp~

101. Illexsquid(Pete) says:

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.

102. huckepick says: