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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.

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Article 104 | Posted October 2, 2003

Recent Comments

The first answer that came into my mind was, "Because most modern men aren't square." Does that qualify me as logical? By the way, what the f... is a manhole cover, actually?

Posted by: Holly Johnson at April 9, 2006 02:30 PM

The shaft leading up to the manhole is round and as we all know a round shaped 'tube' is not only stronger than a square shaped 'tube' but it also uses less material to manufacture. Think about why submarines are round shaped, they have pressure all around it trying to crush it. The same principle applies to a manhole. Therefore the covers are round. Plus they have the added advantage that they will not fall into the hole like a manhole of a different shape would.

Posted by: e hansen at May 24, 2006 02:13 PM

I grew up in Nashua, NH and remember manhole covers there were triangular. I was told it was because they wouldn't rattle.
(Should it be personhole cover?)
;-)

Posted by: Moe at May 29, 2006 04:59 PM

My first thought was that if the man in the manhole had to get equipment down there (wires, tools, etc.) the items would not get stuck as easily on a rounded surface. A square surface has corners that items would get caught on. Now that I'm thinking about it...boy am I missing something here!!!

Posted by: Carrie at December 6, 2006 10:53 AM

When I was asked why Manhole covers are round the first thing that came into my head was because men are not square, silly but true.

Posted by: Cecelia Nosa at May 8, 2007 01:35 PM

This is a good question to use in an interview with a future reference desk librarian. They get questions along these lines all day!

Posted by: Jennifer at September 19, 2007 11:42 AM

Why won't a round cover fall in the hole? Seams to me it would unless the circumference of hole is less than that of cover.

Posted by: terry coon at October 27, 2007 08:18 AM

I was asked that today in an interview....lol

Posted by: Erika at April 21, 2008 02:51 PM

A round manhole cover would not fall through its circular opening????

Posted by: Rick at June 2, 2008 03:42 AM

Manhole covers are round simply because the friggen manholes are round. Why would you make a cover different from the opening?

Posted by: mark at July 16, 2008 02:09 AM

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