An old classic this week.  You have a straight-sided glass of orange juice that is filled to the brim like this…..

…and would like to share it with a friend.  You have no other drinking vessel, and nothing with which to measure the depth or content of the glass.  How do you make sure that you both get a fair share?

As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took.  Solution on Monday.

And while you are thinking about it, here is a terrifying video (via@rpaulwilson)

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

76 comments

    1. I had exactly the same reflexion :
      * It’s obvious
      * Damned, wait : it’s in 3D, and it’s a cylinder !!
      * Ahah, i found the trap…
      *…
      * Hum ??? It doesn’t change anything : it’s still the same obvious solution….

      And while I’m writing on this post, I’m not a real fan of the video…
      I don’t like such a simple brutal scene exposed…

    1. because it says in the post “As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took. “?

    2. Why does this happen in every Friday Puzzle? Richard specifically tells people to post how long it took them to answer, and then some schmuck insults everybody for doing exactly that.

      Don’t feel bad, Hugh, I didn’t figure it out yet either. Doesn’t mean I have to berate others to compensate.

    3. Presuming that this is a troll, and huge arse is having some fun, but I’ll play along.

      ‘I really do not see the point of posting “I’ve done it” comments. What a bunch of sad, lonely people’

      Are they worse than people who have no reading comprehension skills?

  1. You get a stick and beat your friend on the head until he/she says it’s fair?

    (I did get a more reasonable solution a couple of seconds after this solution came into my head…)

  2. As long as rock-paper-scissors is involved, the outcome is fair AFAIC. 😉 Actually, because I had to work this one out IRL once for a can of railroad spikes, I had the answer lodged in a deep memory crevasse and was able to retrieve it with a minimum of effort. But I do remember that it was an “ah ha!” moment when I figured it out the first time.

  3. As long as rock-paper-scissors is involved, the outcome is fair AFAIC. 😉 Actually, because I had to work this one out IRL once for a can of railroad spikes, I had the answer lodged in a deep memory crevasse and was able to retrieve it with a minimum of effort. But I do remember that it was an “ah ha!” moment when I figured it out the first time.

  4. I looked at the questions and though urgh and then looked at the pic and answer jumped out so about 5 secs for me.

  5. There’s one pretty obvious solution that doesn’t involve any additional equipment whatsoever – the only proviso being that both you and your friend have similar sucking capacity.

    However, it’s probably not what Richard’s looking for.

  6. yes it’s easy!
    i knew it as soon as i read it. …
    you come across this almost everytime you drink from a glass.
    but to be absolutely sure i also checked it mathematicaly by calculating an example, that took me a few minutes to write it down

  7. I think the answer is not to be such big babies and worry about one person getting a few millilitres more; the first person should just drink what looks like half and the other should finish it off. But I’m going first 😉

  8. I think to make it more realistic, the problem should say “sibling” instead of “friend”. Friends would just estimate & be done with it. SIBLINGS, however, require ultimate precision. But then, of course, a method to make sure they don’t have to drink from the same cup would be needed, because, y’know… cooties.;-)

    I’ve come across this before, so my solving time doesn’t really count.

  9. The obvious solution requires one share to be consumed before the other comences so I’d abandon that in favour of just passing it back and forth one glug at a time.

    1. Though ANY amount I share with my friend is “fair,” including not sharing any at all. That’s MY orange juice.

  10. My word, everybody seems a bit cranky today. I know I am, because (a) I’m not even supposed to be drinking OJ, and (b) that video is damned upsetting.

    I know I must have seen this puzzle before. It seemed trivially easy, and I’m not that clever.

  11. 20 second.I didn’t managed to figure out the answer at the first in instant, but when I saw saw a comment”3 second” on the top, I suddenly know the solution, pretty obvious,

    I’m slow :(
    seemed like most people got it within 10 seconds

  12. I couldn’t figure it out yesterday, so I came back to it today, and got the answer almost instantaneously. Then I thought it might be different in 3d, but then I figured the answer is the same

  13. If it is fairness that is the issue (ie all parties can agree that the share they get is a fair one) then a simple solution extends to any number of sharers

  14. Presuming they go sequentially, the question becomes, after the first person takes their “equal” share, is the glass then half full or half empty?

  15. I think I got this one right away. Not a whole lot to think about really unless, of course, you bring into account whether it is fresh squeezed or pulp free. Pulp does cling to the side of the glass and who can say how much?

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