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?
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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.
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"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
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 AMThey 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 AMHehe, I was asked this question in an interview and it totally threw me off. I had no idea where to begin so I started with the closest thing I could think of. Here's what I said:
The underlying sewers are all round. The pipes are all round. The round shape is chosen because a circle is the strongest shape for supporting the ground above it, stronger than a square or triangle.Therefore, since the underlying pipes as well as the pipe leading to the streets are round, the manhole cover would be round.
It was a random answer I thought up on the spot.
23-Apr-2010
Posted by: Moe E. at April 22, 2010 9:34 PM
Just had a quick read through the comments and I'm amazed that my first thought doesn't match anyone elses. Corners promote cracking and subsequent failure. Curves on the other hand do not. I was also surprised the most common answer was that a circular cover can't fall in. That would be my last one. Think about the work necessary to move a square/rectangular manhole into a vertical position to enable it to fall through. It's not something that could be done accidently by a vehicle. Finally I love the one about rolling the manhole being easier. That's a great one. In summary my comment above pretty much proves the point of your article lol.
Posted by: John Withers at May 8, 2010 6:52 PMYou can drill a square hole!!!
Posted by: mike at May 16, 2010 5:54 PM