It’s the Friday puzzle!


ignite_knight hospitaller 2A square medieval castle on a square island is under siege. All around the castle there is a square moat 10 meters wide. Unfortunately, a group of  raiders have brought footbridges that are only 9.5 meters long. The invaders do not want to fail in their assault.  How can they use their footbridges to invade the castle?

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.


88 comments on “It’s the Friday puzzle!

  1. TS says:

    Easy, similar to the “circular pond with an island in the middle” problem.
    No urinating your way out of this one folks.

  2. horuskol says:

    done – several seconds

  3. fpilot says:

    Perhaps they can paddle them across.

    Actually if the castle forms the inner side of the moat and rises vertically from the moat (which is how I recall seeing pictures of moats around castles) then I don’t think it can be done.

    I also think that the solution that the puzzle setter has in mind doesn’t take into account the integrity of the outer side of the moat, that is, if it is just dirt and the raiders are being shot at then they would have problems.

    Finally, what about the monsters in the moat?

  4. Rob J says:

    What they need to do is freeze some urine, and attach these blocks of the frozen urine to either end of the walkways, thereby making them longer.

    But yeah, I got it before finishing reading the question. Assuming a few little things, but yes.

  5. Simon says:

    Would soaking the footbridges in urine cause them to expand that extra 0.5 metres?

    Seriously, about 0.5 seconds. 🙂

  6. Martin says:

    Solution in principle – immediate. Alternative solutions within two minutes

  7. Mchl says:

    Got in less than 10 seconds, with two solutions (one of them not really good, but could work 😛 )

  8. noamgr says:

    10-20 seconds after starting to think about it.

    All you have to do is urinate on the… O, it’s been done to death already. Dammit, and I thought I was so original 😉

    I can only think of one solution though. I’m curious about what other solutions you guys came up with.

  9. Frankie says:

    Had an answer immediately after reading…. not sure if it’s right though!

  10. Wolfgang says:

    I think it could be done with four footbridges.

  11. Livi says:

    solved in seconds

  12. carmenego says:

    50cm isn’t a particularly big gap to jump… just saying.

    • uksceptic says:

      Presumably the other end of the footbridge wouldn’t stay there if it wasn’t resting on something at the other end.

      Sorry Carmenego!

    • carmenego says:

      Yeah but if it’s a moat, the water’s not going to be flowing particularly fast. You lat it on the water, and get your trusty valet (read: pawn) to secure it using rope or shoelaces (as someoen else said). Or just tie his feet to the end of the footbridge, get him to hold on to something at the other end and just use his back as an extension?

      Also, aren’t footbridges really easy to make? You just tie a load of wooden planks together with rope. What’s to stop you taking another footbridge apart and reusing the pieces?

      Am I thinking about this too laterally?

  13. christian says:

    I took me a minute of thinking. 2 footbridges wil help them, I hope 🙂

  14. Jim T says:

    Got an answer fairly quickly – few seconds.
    Not sure how ‘good’ it is, though.

    • Paul Moran says:

      when are the answers to these puzzles ever ‘good’ everyone seems to think they need to overthink the solution.

  15. grthink says:

    Yeah, I think I’ve got this. 1min, maybe?

  16. daveman says:

    easy – under a millisecond – solved this well known problem before.

  17. Rob O. says:

    There wasn’t any mention of whether the moat was filled or how deep it is…

  18. Paul says:

    Again another puzzle that is “too open”, and urine or not the good folks here will think of at least 5 ways of solving it. I can think of a couple instantly – although I’m wondering if the ‘squareness’ is an important factor?

    • It’s not open in the same way the last one was; there is no need to invoke deus ex machina solutions. That said, I’m laughing like hell at all the urine references this time around! 🙂

      For the classic answer, yes, squareness (or, rather, being a polygon of certain measure) is an important factor. I heard the puzzle before, so I knew that answer immediately. After thinking about it for a couple minutes, I figured out a three plank solution that works just fine on a round moat as well.

  19. Well invading is pretty mean so I think they should just say sorry and go home!

  20. Rallyannie says:

    Got it by the time I’d finished reading the question.

  21. shpalman says:

    An answer occurred to me within a few seconds.

  22. Tom says:

    Got simple easy answer by the time I read ‘9.5 meters long’. 2nd solution is obvious but very awkward to set up and use.

  23. Just F says:

    9.5 seconds, with only two footbridges and about 20 shoe-laces.

  24. Sandy says:

    I’m blonde and haven’t got it after 5 minutes…. Let you know later.

  25. MShades says:

    First we build a large wooden rabbit….

    Got a couple of solutions, neither of which really feel right. Will have to ponder….

  26. uksceptic says:

    I think I got this one.

    Got it while reading the comments, it just popped in my head!

  27. Vern says:

    Took a few mins to stop thinking up goofy answers and be serious. Moments later my imaginary raider army was scaling the walls.

  28. Rob says:

    I don’t think that any invaders stupid enough not to even do the basic background planning like working out how wide the moat is deserve to succesfully invade the castle so I aint telling them my solution!

  29. lakritze says:

    I’ve got two solutions, one requiring very good high-jump skills. And then there is a third, depending on the existence of extraterrestrial life.

  30. It came naturally, got it before I started to read the question 😀 (It was like “So?.. Oh, that’s what’s asked..”) But to check whether it’s really really right I needed a bit more time.

  31. Michael K Gray says:

    My immediate sub-conscious response was the word:

    Now I have to work out just how to fit that engineering concept into a solution…

  32. Sruixan says:

    As long as the bridges are pretty sturdy, then yes, it can be done without urine.

  33. Got this one straight away, just ’cause it’s a variant of a problem I’ve heard before.

  34. arthwollipot says:

    Unusually, I got this one in just a couple of seconds.

  35. henrik says:

    Sure the moat is square? Easy

  36. Atticus says:

    Are the footbridges made of wood?
    Wood floats… Problem solved.

    I think :p

  37. Got several solutions by now. Took me 61 seconds to come up with two.

  38. Marco says:

    I like the idea of the square moat. If it had been a round one the same solution might have worked, but it would depend on the radius, right? For the square one the length of the square does not matter which makes for a better puzzle. Brilliant.

  39. SilverMarc says:

    Only two bridges needed, solved immediately.

  40. Jerwah says:

    4 answers in a couple of min 1 I’m pretty sure is the intended one none of them require urea in any form

    • Pim says:

      Well if u collect all the urine of your man, and make a catapult out of the bridges. Then shoot is all in the castle maybe you can force the enemy out.

  41. James says:

    Took me a couple of seconds after finishing the article. I think it may be the two plank solution others are thinking of. However, 8 planks would be better, and extra planks wouldn’t hurt.

  42. Took quite sometime to decide the right answer.. too many answers came to mind.

  43. Sidebernie says:

    Less than a minute, although I haven’t verified the math.

  44. Tom Rhoads says:

    I think I gt it. I either got it right away (if I’m right) or not at all.

  45. Sander says:

    Got multiple solutions, but the first (and probably the ‘correct’) one took me only a couple of seconds.

  46. doug says:

    Got it. Took about 10 seconds. Certain I have the correct answer.

  47. Janine says:

    About 3-4 seconds, an easy one I think, that’s if I’ve got it right of course! 🙂

  48. Austin says:

    Solved in a matter of seconds. Let’s hope I’m right.

  49. Roland says:

    The solution “wait for the moat to freeze” reminded me of the following joke:

    Joe heard a rumor that his father, grandfather and great-grandfather had all walked on water on their 21st birthdays.

    So, on his 21st birthday, Joe and his good friend Brian headed out to the lake. “If they did it, I can too!” he insisted.

    When Joe and Brian arrived at the lake, they rented a boat and began paddling. When the got to the middle of the lake, Joe stepped off of the side of the boat… and damn near drowned.

    Furious and somewhat shamed, he and Brian headed for home.

    When Joe arrived back at the family farm, he asked his grandmother for an explanation. “Grandma, why can I not walk on water like my father, and his father, and his father before him?”

    The feeble old grandmother took Joe by the hands, looked into his eyes, and explained, “That’s because your father, grandfather, and great-grandfather were born in January… you were born in July, dear.”

  50. Stuart says:

    I’m not entirely sure how you can have a square moat and yet have it be 10 metres wide. It is either square, in which case width at the corners would have to vary, or it is 10m wide in which case it would have rounded corners and therefore not be square …

  51. FrankN.Stein says:

    couple of seconds… being aware of the basic princpile needed… (If you don’t know that and have to figure it out, it may take a while.)

  52. Amie says:

    Solved as I was reading.
    Few years ago I went on an adventure holiday, they gave us two planks that were shorter than the gap and told us to get across without touching or falling in the river.
    So that’s where my solution comes from! 😀

  53. Travis says:

    Yeah, I got this one immediately. Too much of a classic.

  54. Rebecca says:

    Think I’ve got it. If I have, it took me about a second.

  55. Graham says:

    4 possible solutions came to mind pretty quickly.

  56. Notatheist says:

    5 seconds maybe?

  57. 3 minutes, then it came to me like lightning. Why are we storming Doctor Frankenstien’s castle again?

  58. Jason says:

    I saw a simple 2 plank solution that seemed logical as soon as I scribbled a quick diagram of the setup on a piece of paper. Of course if the raiders were stumped and they’d brought plenty of planks they could always build a giant horse and leave it by the drawbridge…

  59. Amit says:

    two footbridges to attack 🙂 few seconds is all it took.

  60. Rex says:

    TOok me a few seconds.

  61. tvarsa says:

    If the soldiers have run out of urine, why not throw a few thousands of planks into the water instead.

    Oh well, I thought of the 2-plank alternative while I was still reading the problem.

  62. T9sus4 says:

    Nooo, Please delete my post above, I wasn’t meant to give the solution. Please delete, please!

  63. CJ says:

    Send for a certain Greek gentleman ! People’s first thought may not be quite right. Solved in 10 minutes .

  64. rolak says:

    Known for about 40 years 🙂

  65. Cosmin Romania says:

    20 seconds. But I don’t really think I got the correct answer.

  66. CJ says:

    I was assuming the bridges were only 9 metres long which makes for a more interesting puzzle.

  67. Bill says:

    Got it in just a few seconds.

  68. Bucket says:

    I got a solution very quickly, but I’m 99% sure it’s wrong…

  69. Sally says:

    Pole vaulting!!!
    Or swimming, using a soldier as the second half of the bridge, draining the moat, lobbing footbridges at the enemy until they beg for mercy and let the drawbridge down, catching a herring from the moat and using it to fell a nearby tree…

    No idea about the correct, geeky answer though!

  70. shpalman says:

    In the solution I have in mind, the minimum length of plank is sqrt(8/9) of the moat width, so for 10 m moat any plank length greater than 9.43 m would be enough.

  71. Marek says:

    have complely no idea. did you solve it by creating something like bridge with two stable points on the ground between the moat (like pontifex?) ?

  72. Michael K Gray says:

    It dawned on me after a day of not thinking about it!
    It took me about a minute to mathematically confirm it as a viable solution using Pythagoras’ theorem.
    The planks could be a touch shorter than 9.5m and still be viable…
    (In fact, exactly = 20*Sqrt(2)/3 metres; approx 9.428m)

  73. Onur says:


    we can use T shape for solution.

    A bridge is set horizontally to the land, and the others set up perpendicular to it like a up-down reversed T !

  74. Adam says:

    Approach the main gate under the guise of footbridge salesmen. If they ask why they would need footbridges, tell them you’ve surrounded their moat with another moat 9 meters wide.

  75. Joaquin says:

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