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So Just Why *Are* Manhole Covers Round?

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Summary: Interviewing for technical positions can be tricky, but it can also be fun. Expect odd questions, and plan on being able to think on your feet.

So Just Why *Are* Manhole Covers Round?

Well I'll tell you, but if all you want is the answer then you missed the point of the question.

And why am I talking about manhole covers anyway? Well, to be honest it's an excuse to plug my newly published ebooks of interviewing tips for both interviewees and interviewers: 10 Quick Steps to Interviewing for Tech Jobs and 10 Quick Steps to Interviewing the Tech Jobseeker, both now available as PDFs or MP3s at 10quicksteps.com.

"Why are manhole covers round?" is a now famous interview question considered anywhere from thought provoking to downright stupid by people on the receiving end.

Why do interviewers ask it? Well, it's so well known now that they shouldn't anymore but originally it was to see how interviewees responded. It's a way to get a sense for an interview candidate's ability to think on his or her feet outside of their area of expertise. The way the candidate approaches the problem, thinks of and presents potential solutions, defends them, or discards them are all relevant to the interviewer.

It's a simple question. There are simple answers. But the great not-so-secret about these types of questions that everyone seems to miss is that it's not about the answer! The interviewer cares much more about how you arrive a whatever answer you get than what that answer is. Are you thinking logically? Is there method or madness to the solutions you consider, accept, and discard? Are you even willing to entertain such an absurd question?

Oh. You still want the answer.

The most common answer is simply that a round manhole cover can't fall into the hole it covers. But I did tell you that there's more than one answer. Heavy, round manhole covers are easier for one person to move by standing it on end and rolling. Also, you don't have to "align" a manhole cover to it's hole. Any orientation will do.

My father, a mechanical engineer, claimed an answer I hadn't heard before: that round manhole covers are easier to manufacture. That's a nifty answer and it would have done great in the interview except he also claimed that was the only reason and all the others were wrong. Whoops.

And for the record, round is not the ONLY shape that can't fall into its hole. There's a class of such shapes referred to as "Reuleaux polygons". The common characteristic is that they share a constant width or diameter.

I interviewed hundreds of technical candidates in my years at Microsoft. If you're going to be interviewing for a job, 10 Quick Steps to Interviewing for Tech Jobs will give you some insights on what the interviewers are really looking for when they talk to you. If you're an interviewer, 10 Quick Steps to Interviewing the Tech Jobseeker will give you some tips to help you identify and hire that star candidate you're looking for. Both are available now at 10quicksteps.com.

Article C1846 - October 2, 2003

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Recent Comments
45 Comments

Leo

As your father claimed they were easier to manufacture round, and that suggestion was the *only* reason begs the question *why* are they easier to manufacture that way?

Any idea?

Posted by: Just J at September 18, 2009 6:18 AM

the simple answer is round hole square peg.....
other answers ease of replacing in hole ..equal size means no manouvering to match hole .it simply drops in.
there are many answers to this question.......

Posted by: john neil at November 13, 2009 6:02 PM

man holes are round for may reasons,
safety- they cannot fall through their own hole.
cost- they use less materials than other shapes because it's circumfrence is smaller.

Posted by: mads at December 4, 2009 10:50 PM

Round manholes covers will not fall in as a lot of people have commented above and so will other shapes but from the engineering point of view there is one advantage the round shape offers that other shape cannot match round shape does not have sharp corners that could pop out and injure animals in the days of the cart or damage cars today it also is less likely to have corners that break away thus requiring the manhole to be replaced as a triangle or sqaure shape would

Posted by: Arvin Kumar at December 6, 2009 10:38 AM

They are round for ease of use, but the best reason is when a heavy object such as a car drives on top of them the weight of the car is evenly distributed along the outer edge of the manhole, thus equalizing the pressure on the whole unit. A square or rectangle will have uneven pressure on the edges and will fail over time.

Posted by: Richard Armstrong at December 9, 2009 5:45 AM

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