If Richard Feynman applied for a job at Microsoft

Interviewer: Now comes the part of the interview where we ask a question to test your creative thinking ability. Don't think too hard about it, just apply everyday common sense, and describe your reasoning process.

Here's the question: Why are manhole covers round?

Feynman: They're not. Some manhole covers are square. It's true that there are SOME round ones, but I've seen square ones, and rectangular ones.

Interviewer: But just considering the round ones, why are they round?

Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology.

Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones?

Feynman: 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.

Interviewer: Can you think of a property of round covers that gives them an advantage over square ones?

Feynman: We have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it. Also, the term "manhole" implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section. So a cylindrical pipe is the natural shape for manholes. The covers are simply the shape needed to cover up a cylinder.

Interviewer: Do you believe there is a safety issue? I mean, couldn't square covers fall into the hole and hurt someone?

Feynman: Not likely. Square covers are sometimes used on prefabricated vaults where the access passage is also square. The cover is larger than the passage, and sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. Let's assume a two-foot square opening and a ledge width of 1-1/2 inches. In order to get it to fall in, you would have to lift one side of the cover, then rotate it 30 degrees so that the cover would clear the ledge, and then tilt the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. Yes, it's possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely. Applying common engineering sense, the shape of a manhole cover is entirely determined by the shape of the opening it is intended to cover.

Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.)

(Interviewer returns after 10 minutes)

Interviewer: We are going to recommend you for immediate hiring into the marketing department.

Keith Michaels

Comment Feed 45 comments on this post


Feynman was da bomb! May his soul rest in peace.

Tuesday, Jun 22, 2010, 5:49 AM


Thank you! I hate that kind of pretend-smart questions with a vengeance, and I hope that some day a truly smart person comes along and gives a well thought through answer for once.

Wednesday, Nov 3, 2010, 2:43 PM


All that being said, it's actually fairly easy to accidentally drop a non-round grill or cover into it's pit. Speaking from an urbex group's experience.
This is because they are generally lifted using hooks that fit in the lift-points. Which may be off-center, and can slip free if the hook or cover rotates.
It's generally really, really hard to get them back out of the pit.

Sigh. I wonder if the entire mortgage/CDO/toxic-derivatives mess could even have started if Feynman was still alive to ridicule such foolishness?

Wednesday, Nov 3, 2010, 3:24 PM


What we have is a world full of experts. Real.

Then we have a world full of experts. Wannabe, but no real clue. The ones that think that reading a wiki puts them on par with someone with a postgraduate degree in a particular specialization.

Then you have the people in the middle (often in marketing). Who think that there is a need to broker relationships betwen either ends, and try to give the semblance of an indication that they understand the technology, expertise of one end, and empathize with the requirements at the other end. Usually, they screw up both, and just modify the signal to noise ratio.

Wednesday, Nov 3, 2010, 4:04 PM


Interviews are to biased. I mean small puzzles solvable in a few mins may demonstrate you are very smart but I dont think it ever demonstrates your problem solving ability. I think one should simply test one on data structures and the whole day give a real life s/w design problem to build with lots of points to improve built in.

Make people sit in from of computers for 4-5 hours and give them a partially complete problems, say with all the boiler plate code mostly done and then ask the user to try and fill in/improve on 3-5 parts as best as they can.

I dont know if interviewers dont do this because its too hard or because they dont really expect people to be that good...
Its full proof and tests weather you can actually find the problem and understand it enough to actually implement it and test it there and then.

Wednesday, Nov 3, 2010, 5:09 PM


Feynman: Manhole covers are round so you can spin them on a smooth surface thus inspiring you to calculate the rate of descent of their centre of gravity and go on to win a Nobel Prize. Can I have a job in the cafe? Plates are so much easier.

Wednesday, Nov 3, 2010, 6:28 PM


Mostly interviewers are generally unqualified and end up asking questions which they would never have been able to solve if they were being interviewed. I was designed and created a social networking site which has around 50m users and have done extensive work on high scalability. I recently decided to move to US and was interviewing with a few companies. I had an interview with Microsoft (which I was not keen on joining anyways) - and an interviewer asked me to write code for differences in Org chart on a stupid collaborative editor of their own. I wrote the code and then he kept pointing out silly things like you missed a semicolon here, you missed a comma there. After almost 1.5 hours of interview I said that I am not interesting in taking it forward and ended the interview. The person interviewing me was one of 30k engineers in Microsoft and I doubt microsoft has more than a few hundred smart engineers. All others are just run-of-the-mill engineers who have no clue of the bigger picture and have done nothing more than code if-then-else constructs based on PM specs in their life.
And you won't believe, I had similar experience interviewing with google a few years back. Google is full of nerds looking for more nerds who are so away from reality, which is why they do not have a single successful social product. Who the hell is expected to solve research problems in interview. Researchers took years to develop them and either you know them or do not know them. All Google is looking for is a repository of CLR rote learners who want to be extended school for rest of their lives.
Facebook is also going the same route. If you look at their engineering puzzles, its full of problems based on BronKerbosch, KnapSack, PerfectMatch, Trie, DP problems - most of them standard algos which can be found on wikipedia if you know what you are looking for. The company is no longer looking for entrepreneurs but run-of-the-mill algo coders. This disconnect will just keep talent away from that company as soon as the IPO is done.
I am glad that I got an offer from an awesome company where this disconnect is not there. They were focussing on what I can do and gave me a chance to prove what I can do in a real job. I was still undergoing interview process at Facebook while I got this offer and decided to ditch Facebook to join this company and am looking forward to it.

Wednesday, Nov 3, 2010, 9:26 PM


wish I had known this blog a few weeks back. no issues. one more answer is that circular is the only shape where you can remove it easily if a lot of mud and other stuff accumulate over it (say after a flood)- without having to clean the corners first to remove it.
more than once I've been asked questions in interviews where the answers that were finally given contradicted a few assumptions in the question itself. Anyway, I got the above mentioned job. I still hate any kind of tests or exams that are time based or have a pressurized environment.

Wednesday, Nov 3, 2010, 9:50 PM


if the interviewer has a twang in his voice the answer would have been: safety.

Wednesday, Nov 3, 2010, 11:40 PM


Microsoft, eh? Big Deal it's so over rated, so last decade

Wednesday, Nov 3, 2010, 11:52 PM


Maybe I'm not getting it, but surely you can have a round manhole cover for a square hole and vice versa? As long as the cover is big enough to cover the entire hole, the shape can be whatever you want it to be, surely.

Wednesday, Nov 3, 2010, 11:55 PM

John P. Qublic:

I agree with Sid. Although for Richard Feynman I think he would be thanked before they got to the lunch interview. Unfortunately many of the questions are asked by people that can only read the answer or be trained for the right answer. Puzzle questions are in theory supposed to show your thought process, and getting the right answer is not the point...but it has degenerated into getting it right or not. A pass fail system of puzzles and code problems.

Check out:



John P. Qublic

Thursday, Nov 4, 2010, 12:02 AM

Kevin Morrill:

I worked at Microsoft for 10 years. After sitting in on well over a hundred interviews with 5 other interviewers, I never once saw this question asked. Not sure where it gets its continuing reputation.

Thursday, Nov 4, 2010, 12:37 AM



it got its reputation from Google!

Thursday, Nov 4, 2010, 1:30 AM

Alex K:

Surely the real reason manhole covers are round is that it makes htem easier to use in fake ufo photos?

Thursday, Nov 4, 2010, 4:02 AM


I know some people are bashing interview techniques but in defense of the interviewer they have to live with the person they hire. When I recommend someone to be hired, I have to work with them, I have to manage them, and if they can't do the job or can't work with my team I also have to fire them. Put that responsibility on top of having to do the same interview 20 times and asking the same dumb question 20 times (do you really think I care what the basic principles of object oriented programming is anymore) and I would rather do ANYTHING than conduct another interview with a person whose resume shows that they have 12 years experience but can't explain what a singleton is or walks out of the interview when I ask them to write code on a piece of paper. Unfortunately if I don't ask these questions (that anyone who graduated college should be able to answer) I will have to fire them and that's worse than going through this process.

A few simple tips for those people being interviewed:
- It's just as miserable for me as it is for you.
- I'm looking at more than your answers, I'm also looking at your personality and how you react to me and the questions.
- If you dislike the questions I'm asking then you will likely dislike the company culture too.
- If you decide to end the interview early be nice about it because like I said in the first point, it's just as miserable for me as it is for you (and thank you for saving me some time to get some real work done).

Thursday, Nov 4, 2010, 7:40 AM


Maybe from this book:

Thursday, Nov 4, 2010, 8:50 AM

Einstein was a clerk.:

He probably got the manhole question

Thursday, Nov 4, 2010, 8:59 AM


I hear you Tom. But not only the candidate should be sweet to you but they have to be able to deliever some results. If your interviews will consist exclusively of manhole-shape type questions then you are at risk to end up with a team of very polite professional plumbers.

Thursday, Nov 4, 2010, 9:05 AM


Manhole covers are round so that they cannot fall into the hole and kill someone. Our sanitary sewer covers weigh about 160lbs, not something you would want to have to haul out of the holes on a regular basis either. Most times they're opened up, it's not for a person to actually go in anyway, that's avoided as much as possible since the atmosphere has to be tested, extrication set up, etc...

The only non-round ones I've ever seen are actually hinged hatch lids for something like a prefab pump station.

The shape of the cover doesn't really have anything to do with the shape of the vault either. Even rectangular vaults have a tapered section at the top top that then has a series of rings that can be stacked to achieve the proper height for the cover to match the eventual level of the pavement.

Older lines are a bit different in that they will often have built in place brick vaults.

Thursday, Nov 4, 2010, 9:25 AM


Alex, I do not normally ask the manhole question (thankfully), but do have to ask the 20 or so questions that always astound me when people get wrong along with having them write a function on a piece of paper (one of the most useless things in the world). If (and only if) the person doesn't show any personality during an interview do I ask them a non-programming question to get them talking but that's to listen how they explain things and not that I care what the answer is.

Thursday, Nov 4, 2010, 9:54 AM


And having worked for a major metropolitan power company, the real answer is two-fold. One it is physically impossible for a round manhole cover to fall through its opening. The second reason is a very practical one. Since manhole covers are so heavy, it is easy for a single individual to pry it open and roll it away from the opening without having to carry it, drag it etc.

Thursday, Nov 4, 2010, 10:48 AM


Isn't a valid answer that it's easier to place them because the angle of rotation doesn't matter?

Thursday, Nov 4, 2010, 11:03 AM


"Yes, it's possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely."

Then he's wrong and being just as much of an ass as the fictional interviewer. "Just train the user" is a bad fallback answer and anyone with half a clue knows the massive assumptions behind that statement

Thursday, Nov 4, 2010, 3:04 PM


Finally a smart answer to a dumb question :)

May I suggest a new question for Microsoft interviewers?

Why are corners of Windows in XP blunt instead of sharp?

Thursday, Nov 4, 2010, 6:54 PM


I am not sure if anyone has mentioned this BUT, you cannot lose a round person hole cover down the hole it covers.
It's just simple mechanics.
best wishes.

Friday, Nov 5, 2010, 3:07 AM


If you want a good interview question, consider this from my Broadcast TV days.
If TV pictures are square, why are round lenses used?

Actually it's not a trick question. Consider the Anamorphic lens used in widescreen (rather than "cinerama")
Nothing to do with programming, so I will understand if this comment gets moderated out.
best wishes

Friday, Nov 5, 2010, 3:12 AM

Abhiram Natarajan:

Nice to way to vent your frustration of having been rejected at Microsoft.

Friday, Nov 5, 2010, 4:08 AM

Jeff Gonzales:

He's got electrolytes!


Friday, Nov 5, 2010, 2:00 PM


Where exactly are there square or rectangular manhole covers? And the interviewer could easily recover by asking "Why are the vast majority of manhole covers round?"

And I don't think the concern of dropping the cover down there is really a safety hazard, as if there is someone is down there you shouldn't be doing anything with the cover (you're not removing it as it had to be open for them to go down, and you would wait for them to return before you cover it up again). But dropping it down there would still be a pain to retrieve, which I believe is the real reason (assuming that Encyclopedia Brown story I read as a kid didn't lie to me).

And while you can make a quadrilateral cover that could not fall below by giving it very thick rims such that the diagonal of the hole is smaller than either side's width, its much easier to do with a round cover.

Friday, Nov 5, 2010, 2:15 PM


I think you should mention that this article is adjusted from an old joke.

Saturday, Nov 6, 2010, 5:23 AM


A question that I used to use 40 years ago for hardware R&D engineering hires was:
"What causes the tides?"
Usual cocky reply, "Easy, the moon"
"The moon circles the earth once a day. So why are their two tides a day? Discuss".
Almost nobody knew the real answer but most would talk it through and thus disclose their flexibility of thinking. Appropriate for R & D.
( The answer is that the earth and moon rotate around each other as a body pair with the center of spin close to the edge of the earth. The tide on the side of the earth opposite to the moon is caused by centrifugal force of the spin which just happens to be about double the amount of the moon's gravitational pull, thus giving a nearly equal tidal rise by chance. The anomalous 4 tides a day in Southampton, England is a local matter ).

Saturday, Nov 6, 2010, 6:58 AM


I believe that a triangular cover could not fall down its hole (assuming a triangular hole) - especially since there almost has to be a lip around the edge. That reason is, at best, secondary.
There are a number of very good reasons to make the hole round (as in the story) and that would have to be the primary reason to make the cover round.
As others have mentioned there are a number of good secondary reasons.

Why would anyone assume that there is only one right answer to a real life question?

Sunday, Nov 7, 2010, 2:11 PM


It is a simple matter of saving money through efficient use of materials.

Substitute [unit] with whatever units of length you prefer:
A square cover 3 [unit]s across takes 9 square [unit]s of material to make.
A circle with a 3 [unit] diameter requires approx 7.07 square units of material.

A round cover uses less material to cover the exact same width of hole. It is a simple and efficient design.

Thursday, Nov 18, 2010, 12:05 PM


I have interviewed hundreds of techies for techie jobs over the years.
Oh. I am a techie.

In the first 15 or so years I hired them - or not - using my own judgement.

In the last 10 years we were owned by a large corporate and everyone had to be interviewed with eaxctly the same script.

Two of the questions were these "puzzle" nonsense things.

"How many piano-tuners are there in London" and "If you could be any animal you liked; What would it be and why?" Never did find out the purpose of these questions - I wasn't looking for an animal piano-tuner. Already over-staffed in that department. Took on loads during the animal piano-tuner downturn - they were cheap.

Anyway, towards the end of the interview you give the interviewee an opportunity to expound on their knowledge of the company - see if they've done their research - see how interested they are etc...

Then you finish with something like "Do you have any questions for me?"

One candidate said "Yes. If you could be any animal you liked; What would it be and why?".

I hired him on the spot. Think I said "Wolf".

Friday, Nov 19, 2010, 2:23 PM


Ever try to roll a square?

Sunday, Nov 21, 2010, 12:35 PM

marvin nubwaxer:

round is the only shape that will not accidentally fall into the hole.

Tuesday, Nov 30, 2010, 9:29 PM


A Ph.D. or two could probably be mined from the history of manhole covers. "Feynman" almost certainly speaks the truth on certain points of history, but his idea about training the technicians to avoid a dangerous failure mode, rather than fool-proofing the hardware, proves that he should never be let anywhere near an engineering job.

(My advice: stick with particle physics!)

Whatever the reason circular covers were first hit upon, once it was demonstrated they had significant built-in safety characteristics, that should have pretty much ended the debate. The only municipalities that should be buying square manhole covers are those that installed square manholes before the advantages of circles came to light.

Tuesday, Feb 15, 2011, 10:48 AM


What really surprised me is notice that Americans make this kind of Question in Job Interviews. Jobs interviews are not like this kind of tests in the rest of the world (well, perhaps in Japan).

In a normal non-American interview, people ask things about the experience and knowledge of the applicant, who is viewed as a responsible adult who can be evaluated based on his/her merits, formation, education, and experience, and not on the result of a single test-like interview.

Saturday, Feb 19, 2011, 3:20 AM

Charles Strahan:

Fascinating. The idea that a circular cover is the only shape that won't fall in is silly. Ultimately, all you need to ensure is that the shortest side of your cover is longer than the largest gap in the opening. Given a square opening, you would make each side of your square cover longer than the diagonal, which can be easily found using the Pythagorean theorum.

What I love above these stupid questions is that they help me as an interviewee judge the interviewer across from me. When I get snickers and cute patronizing facial expressions from the interviewer(s) when I give the "wrong" answer, I immediately know that the company is full of pretentious, closed minded nerdos.

Wednesday, Feb 23, 2011, 11:30 AM



Can a side of a square ever be larger that its diagonal (in Euclidean geometry at least)??

Wednesday, Feb 23, 2011, 11:52 AM


Like Albert Einstein, Feynman produced brilliant theories during a brief period of his life. Both relied heavily on other scientists to help them fill in the missing pieces. Albert created his masterpiece working outside the confines of the university. Both made a fortune off their celebrity by feeding a steady diet of BS to their loyal groupies (NY Times). A good marketer relies on all means available to sell the product including a healthy dose of BS if needed and the good doctors were full of it during the latter halves of their lives. Funny how naive people are to think that because a person is brilliant in a field of physics, music, acting, sports; and achieve celebrity status, that he is also an expert on a range of subjects outside his field of expertise.

Monday, Feb 28, 2011, 10:22 PM


These questions and pretend answers from RF may be good for occasional humor... But I have seen these things on Microsoft employee's blog. I don't understand, why someone write something like this... which is clearly idiotic. You never know what RF would really be his answer to those stupid questions... or would he even bother to answer those questions at all. Assuming, If Feynman really applied for a job at Microsoft... would any engineer really dare to interview him for his common sense and his ability to solve some logically challenged puzzles.

Basically there is no difference between engineer or physicist or astronaut... all have to apply common sense and problem solving ability to solve problems they face in their professional lives...

Its just idiotic...


Thursday, Mar 3, 2011, 12:30 AM


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Sunday, Oct 2, 2011, 1:34 PM

Chris Sells:

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Sunday, Oct 2, 2011, 2:00 PM

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