Well, what a week.  I am in NYC now for a few meetings and a skeptics conference.

Thanks to everyone who contributed to the confession post.  It received an amazing response, and feel free to continue posting your confessions and helping others.

So, to the Friday puzzle….

A Petri dish contains a colony of bacteria. Every minute every bacterium divides into two. The colony was started by just one cell at noon. 47 minutes later the Petri dish was half full. At what time will the dish be full of bacteria?

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. Ah, the old “scum in the pond” riddle.

    Took me zero time to solve.

    If you keep this up I will gain confidence and start to feel intili.. intiegen…. clever!

  2. Pretty sure the obvious answer’s not the correct one, but without pencil
    and paper handy I’m having trouble thinking it through.

    1. you’re thinking too complicated! this is only a matter of understanding the question, there’s no need for writing anything down.

  3. Took me a minute to find the time again to say what time it would be full. I like ones I can do quickly, makes me feel brainy for the day!

  4. About as long as it took to finish reading it. But I cheat by being a physicist who’s used to this sort of problem (though not example)

  5. I am sure that the ‘right’ answer will actually be wrong, because it would only be correct in a liquid culture of bacteria, but not on a Petri dish. On a Petri dish there are space limitations – only bacteria with room about them can grow. Especially on a dish started with only one colony that is now half full, most of the bacteria are stuck in a bulk of bacteria and can’t grow and divide! And by the way, a division time of 1 minute is really unrealistically short.

    Sorry for this pedantic post, but I’m working with E. coli bacteria.

    1. Even in liquid culture it depends how good your shaker is… Not to mention the fact that though your bacteria *should* grow so fast, they are bastard bacteria who want to foil your experiment and make you stay in the lab late because you can’t induce them until you have X amount… I am sure the bacteria somehow know this!

      As for the puzzle, as long as it took me to read it.

  6. If it really took 47 minutes to get half full, dividing every minute, it has to be one hell of a big petri dish, more like a petri swimming pool!

  7. LOL, rhabarber. I was about to post just about exactly what you did, from space limitations to the division time. I spent years working with E.coli.

  8. Knew it, so knew the answer before finishing reading. (although the puzzle over simplifies reality but we can ignore that)

  9. Knew it instantly. The way I remember it presented when I was a kid was slightly different to this, which made it just a tiny bit less obvious. I’ll give it on Monday, cos it kind of gives the answer away if you’ve heard both versions.

  10. D’oh! Came up with the wrong answer in 0 seconds (thinking stupidly along the lines of shooting arrows at tortoises), then used some basic maths to get the right answer in about 15 seconds, before realising that once again there is a logical shortcut!

  11. I like to think I have a ridiculously illogical brain and can never answer these things. I feel a bit upset this one was so easy. But also quite smug too.

  12. Got it straight away. But your I0 I0 II = I0.50 (from the ’59 Seconds’ book) had me going for more than an hour – at which point I gave up. And I didn’t get the teacup either.

  13. it is easy when you are in the right mindset, but it’s a nice test if you get easily distracted by somebody making a big show or if you can focus on the basic points of a problem.

  14. Easy! Get it immediately. Forget the complicated calculations. The key sentence is: “every minute every bacterium divides into two” – this is what is called exponential growth.
    There’s only one problem: we must assume that not a single bacterium dies before replicating itself – that’s not what happens in nature.

  15. This might be more deceptive, at least to the visual thinkers, if you displayed the problem along with a linear graph that left ample room for possible answers. Just pulling the numbers out of a word problem made it trivially easy.

  16. OK, I’ve got this one. It took me about 1 minute after spending some time trying to work out figures on a piece of paper before realizing what the answer is.

  17. This is not as simple at it first appears. Only the bacteria around the edge would be able to grow and divide, so perhaps this problem could have been better presented…

  18. This feels less like a riddle and more like a word problem from my 9th grade math class. Still love the site, keep it up!

  19. I was first introduced to this little puzzle in a talk given by David Suzuki, to indicate where our intuitions about exponential growth fail badly.

  20. Hm … why does no one really post the answer ?
    Anyhow .. I would first think .. I’m not gonna work my brain at all … vaguely seeing lots of difficult and energy taking mind calculations .. and then .. sitting in my not too smart self .. I saw it .. at least .. I thought .. but then, if you start thinking … you know that not all bacteria will divide .. so it will take more than a second .. how much .. reality will show . Ofcourse .. statistics would come up with chances stuff and calculations and bla bla bla .. (you know, the derren brown theory on e.g. lottery predictions .. ).

    Now then .. I’m in a box here and I can’t get out … it’s sunday morning .. and I’m bored .. as usual .. so that’s why I am here .. I guess .. And also because Brown Towers could not keep it up …. cheap server management … ehehe …

  21. Read the top of the page:

    “As ever, please do not post your answer, but feel free to say if you have a solution and how long it took. Answer on Monday.”

  22. Took me about 1 second… but in all fairness I solved the same problem (just with leaves covering a lake) years ago. That time it took me some more seconds 🙂

  23. I’m with everyone else: it took about point zero zero zero … seconds.
    It was so easy I went back to make sure of the details. Was it supposed to be hard??

  24. Just started following your blog/twitter. Like the sound of these puzzles. I know you’ve already published the answer somewhere but I didn’t look and I did get this one pretty much the moment after I read it. It’s not that hard, but still quite nicely elegant…

  25. ummm… I saw the puzzle after this and wanted to see whether the previous puzzle would be as dumb, it seems so. Sorry for being a troll but I dispair at this.

  26. Wow that was eay just took me 2 secs

    ANSWER- if every cell takes 1min to copy it self, so if it half full then every cell will double and 2 halves make a whole.

    one minute later the dish will be full 47mins+1min = 48mins past Noon = 12:48

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