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Why are manhole covers round? What if you get that in a job interview?

en.wikipedia.org — The question of why manhole covers are typically round, at least in the U.S., was made notorious by Microsoft, who began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one "right" answer, the problem exerts a strange fascination on people.

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  • sundancekid503, on 10/12/2007, -3/+264If asked this question in an interview for a large company you should get a disgusted look on your face, then remind the interviewer that this is the 21st century and they should be referred to as "personholes".

  • KF6BBL, on 10/12/2007, -1/+95"Circular covers can also be moved around by rolling, and they need not be aligned to put them back."

    Yea but it drives me crazy when they put them back with the yellow stripes going sideways.
  • Switch22, on 10/12/2007, -3/+77For the lazy in all of us:

    Reasons for the shape include: A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it was inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.) Round tubes are the strongest and most material-efficient shape against the compression of the earth around them. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using horizontal boring mills. Circular covers can also be moved around by rolling, and they need not be aligned to put them back.
  • zumpiez, on 10/12/2007, -5/+60Because Microsoft asks it during interviews for such jobs.
  • SharkJumper, on 10/12/2007, -6/+58I answered, "They're not!"

    I didn't get the job.
  • jo42, on 10/12/2007, -4/+56Q: Why are manhole covers round?

    A: Because manholes are round.
  • Javamancer, on 10/12/2007, -5/+55Are we not forgetting that only one bottle contains poison, hence you'd only need one prisoner? Just give him a sip from each bottle til he drops dead, and there's your poison. Or am i missing something?
  • NSMike, on 10/12/2007, -3/+48I hate you and all interviewers who ask questions like that. Stay out of my soul. I don't want the vaccuum where yours used to be sucking mine in.
  • merreborn, on 10/12/2007, -6/+50"why is this under PROGRAMMING?"

    If you're ever interviewed for a programming job at a big company, you're gonna get questions like this. At Yahoo! they asked me one about how to figure out which one of 1000 bottles of wine was poisoned, using less than 10 prisoners as 'test subjects'.

    I eventually worked it out, but didn't land the job :(
  • natterca, on 10/12/2007, -3/+47i would never hire someone with that answer.
  • csmyth, on 10/12/2007, -2/+44Reminds me of that hypothetical feynman interview. Oh, here it is:
    http://hebig.org/blogs/archives/main/000962.php

    "Yes. Round covers are used when the hole they are covering up is also round. It's simplest to cover a round hole with a round cover"
  • dobey123, on 10/12/2007, -1/+39@ ronabop
    split the wines in half and combine them into 2 glasses. Samples of 500 in glass "A" and samples of the other in glass "B". Keep working your way down until you have 1 wine left. It's a binary-type puzzle.
  • bernierm, on 10/12/2007, -0/+37My answer: "Optimally, they should be cone-shaped, so that a single size cover can fit any size manhole."

    Q: "But what about small manholes? Wouldn't there be a lot of extra manhole cover protruding into the street?"

    A: "That sounds more like a user problem, not a design problem."
  • surfit, on 10/12/2007, -1/+35Round manhole covers are not common in certain locations, where I am they are all square, retangular or made up of two right-angle triangles (because it apparently makes them quiet when cars run over them compared to round ones).

    You may be asking why I know? Well this topic came up on Digg a few months ago and I was sure I wasn't imagining things, so the next day I decided to look at the ground on a 60 minute walk through a town and city on my travels to the bank, and I saw absolutely no round ones on the way. Sad but true. ;)

    So if someone got this question around where I live in the UK, then they would have to ignore the fact that they aren't round before answering the question.
  • Otto, on 10/12/2007, -1/+30WTF is a brink?
  • Otto, on 10/12/2007, -4/+30>>>"If you could be any kind of fruit, what kind of fruit would you be and why?"

    Ham.
    Because fruit sucks.
  • stikko, on 10/12/2007, -2/+25"If you're ever interviewed for a programming job at a big company, you're gonna get questions like this. At Yahoo! they asked me one about how to figure out which one of 1000 bottles of wine was poisoned, using less than 10 prisoners as 'test subjects'."

    Yeah, you COULD keep splitting the sample in half and kill off 8 or 9 prisoners...

    Or you could kill off ~4 like so:
    Give each of 10 prisoners 100 bottles. 1 dies. Distribute those 100 bottles to each of the remaining 9 (1 will get 12 bottles, the rest will get 11). 2 now dead. 12 bottles to 8 prisoners. 3 now dead. Worst case scenario you have to do 1 more round of testing of 2 bottles and you end up with 4 dead.
  • GCRaya, on 10/12/2007, -4/+26That's the problem with the tubes! The square! That's why they are clogged and i didn't get the Internet my secretary sent me.
  • laserdisc, on 10/12/2007, -0/+22These questions are designed to test your intellect. In which case management can assess if you pose a potential threat to their position in the future. That's why in interviews for low to mid range positions you don't want to bring to bare your full potential. In most cases (not all) this has actually worked in landing jobs. Heh.
  • whiskeymb, on 10/12/2007, -7/+28@warpfox

    cause it's typically a question you get when interviewing for a programming position at MS.
  • TheGeek27, on 11/10/2007, -1/+22Because giraffes are not purple.
  • Rikkochet, on 10/12/2007, -1/+20Reminds me of this gimmick a local radio station has, called "Tell us something we don't know."
    They get a bunch of people calling in to tell them some random fact most people don't know, and then give away some tickets. The example they gave one day was just that - why are manhole covers round?
    One lady called and gave some fact about breast size, one dude called about carpet underlay and the third guy was right on the ball.
    "Do you know why they put the manholes right in the lanes on the road instead of in the middle where you don't drive over every one?"
    DJ: "No, why?"
    "JUST TO PISS YOU OFF"
  • inactive, on 10/12/2007, -4/+23You shoulda just said "so they don't crack sewer workers in the skull."
  • oogee, on 10/12/2007, -2/+20A similar approach to disarming an applicant is supposedly used at a medical school in Ontario. Those wishing to enter medical school were asked "When did you last masturbate?"
    I guess applicants who were too shy to answer the question may have trouble listening to patients who complain of embarrassing symptoms.
    Too bad my grades were never high enough for med school, I could have answered that one . . .
  • biffen, on 10/12/2007, -2/+20No way thats true! Is it?

    I demand linkage.
  • wistar, on 10/12/2007, -3/+20No, usually software engineering.
  • subtle, on 10/12/2007, -0/+17>>Formula 1 race cars create so much vacuum due to their aerodynamics that they can lift a manhole cover above the ground. During races on city streets, the manhole covers must be welded down to prevent injury. In 1990, while racing in Montreal, racer Jesus Pareja's car struck a manhole cover that was lifted by the ground effect of the car he was following, causing his car to catch fire.

    So that explains that scene in Sly's crappy racing movie
  • loveandrockets, on 10/12/2007, -1/+18The poet Andre Codrescu was giving a speech at Microsoft HQ. Someone stood up during the Q and A and asked Andre "why are manhole covers round?" The crowd burst into laughter. Without missing a beat, the poet replied:

    "That's easy. In a fight, a round shield is better than a square one. The circle is also a symbol of infinity, which is why church domes are also round. The principle of 'as above as below' reminds pedestrians that they live in a divine world."

    Just goes to show poets think differently than normal people.
  • deadbaby, on 11/10/2007, -0/+16It's true. I first read about it in Alan Deutschman's book "The Second Coming of Steve Jobs" (no, it's not erotic fiction. Just a bad title) Someone had brought in a very straight laced guy for an interview. Steve didn't think he was a good fit so he tried to rattle his cage a little. He also writes about an incident where Steve Jobs made a marketing exec cry after some relentless teasing.

    It's an interesting book. Definitely worth picking up.
  • qwickone, on 10/12/2007, -8/+24That doesnt mean the manhole cover has to be round. my milk bottle has a round opening, but the bottle itself is rectangular.
  • beervolcano, on 10/12/2007, -5/+21Maybe they didn't hire you because you didn't know the difference between "through" and "threw."
  • marthaphoebe, on 10/12/2007, -2/+18They are round because it's easier to roll off and steal and resell to the same buyers. Which is precisely what the manhole cover manufacturers do. This is how they maintain their revenue stream. C'mon Microsoft, you know you do the same thing. Ok, now hire me.
  • subtle, on 10/12/2007, -3/+18bollingj: No, stupid, he RTFA
  • beervolcano, on 10/12/2007, -0/+15101. Being the subject of an interwiew question.
  • anorris, on 10/12/2007, -5/+19@ronabop:

    My immediate reaction: Give the first prisoner drinks from 500 bottles. If he is poisoned, throw out the other half, if he isn't, throw out the half he drank. Next prisoner gets 250, then 125, then 62 or 63, then 31 or 32, then 15 or 16, then 7 or 8, then 3 or 4, then 1 or 2, then 1. Depending on your luck, you have the bottle of poison in either 9 or 10 tests.

    Edit: Dobey posted while I was writing that out...
  • spidoman, on 10/12/2007, -0/+14lol best answer "because manholes are"
  • Vlatro, on 10/12/2007, -0/+14They used to be made out of solid iron, about 3" thick and 30" (Approx.) in Diameter. Iron isn't cheap when you need 2 - 3 of these for every mile of city road. Paying by the pound for the iron, removing the corners results in a 1/5 reduction in weight. That's a buy 4, get 1 free type of sale. Talk all you want about safety, practicality, etc. The decision to buy round manhole covers came from someone working within a budget.
  • conan359, on 11/10/2007, -3/+17This provided me with a great moment. I had a boss in college who liked to prove us wrong, even when we were talking about random *****. Like we saw the old video of the real life(fake) transformer and were talking about how cool it was, he came over and told us it was fake. Good thing cause we had no idea.
    Anyways he found an article about this and then quizzed us with the questions and I gave him this exact answer but he did not believe me and I had to cut example shapes out of paper. He ended up looking like a huge ass in the end. It was a great feeling. Then as soon as he left I went back to sleep.
  • NSMike, on 10/12/2007, -4/+18"Goodniiiigghtt, ah-ding ding ding ding..."
    "I... I don't know what to do when you do that..."
  • MikeSD34, on 10/12/2007, -0/+13Because it shows the person can't think on the spot. The idea isn't to come up with the right answer based on facts and figures, it's to judge your reasoning ability given what you have in front of you.
  • bcore, on 11/10/2007, -2/+15haha pirates of silicone valley... sounds like pr0n. :)
  • MrEdmo, on 10/12/2007, -6/+19the way i would test the wine:
    you have 10 dudes. and 1000 wines. treat the inmate number as a digit and give sips out. IE. give wine 1 to dude 1. wine 2 to dude 2. wine 3 to dudes 1 and 2. wine 4 to dude 3. etc

    then when 1 or more die. use their numbers to construct a binary number.
    IE if dudes 4 and 1 die then you know it is wine #9
  • bobothn, on 10/12/2007, -0/+12http://mathworld.wolfram.com/ReuleauxTriangle.html
  • PoorYorick, on 11/10/2007, -0/+12Guess you had to be there.
  • Otto, on 10/12/2007, -0/+12Ahhh! Brick. Well, the answer is obvious.

    101. Beating three kinds of hell out of interviewers who ask stupid questions during interviews.
  • inactive, on 10/12/2007, -4/+16@spokenrope: "If you could be any kind of fruit, what kind of fruit would you be and why?"

    I would like to be a potato. What do you mean it's not a fruit? Are you racist?
  • inkubux, on 10/12/2007, -0/+11What... is the air-speed velocity of an unladen swallow?
  • frogpelt, on 11/10/2007, -1/+12Heartwarming story, Ted.
  • MrEdmo, on 10/12/2007, -0/+10you can't just use one because you dont know how long the poison takes to kill. if it happens to take a day to kill you would need a possible 1000 days to find the right one. why wait 1000 days when you can do it in one...
  • theone3, on 10/12/2007, -11/+21Pipes are round.
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