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.
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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
A round manhole cover would not fall through its circular opening????
Posted by: Rick at June 2, 2008 3:42 AMManhole 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 2:09 AMBecause they are not square.............
Posted by: Adri Malia at September 17, 2008 3:47 AMBecause round seals in moisture and hence the smell better, than straight edges. The edges would not seal as much during expansion and contraction. Ever wonder why tumblers and dive watches (mostly) are round too? That's why.
Posted by: vivebrulee at October 16, 2008 3:52 AMA round manhole cover will not fall through a round manhole because the actual circumference of the frame it sits in is slightly greater than the circumference of the whole itself, thus allowing it to lay flush with the street without falling into the whole. No matter how you position the cover, its smallest diameter (all diameters) is larger than any diameter of the hole.
Posted by: Bart at December 8, 2008 6:11 PMI too, knew the answer would be simple...because holes are round....more specifically manholes are round...does that mean I'm simple???/
Posted by: Ali Ree at June 8, 2009 10:22 AMWell, I never really thought of that until I read it.
But it totally makes sense, ok round the cover is larger than the opening! So it can't fall in, even if you rotate it or turn it on it's side.
But... If you use a rectangle for example, still the same effect, but if you turn it on it's side or rotate it, it could fall in, or not line up.
Other "Ideas" around the Manhole.
[Content copied from Wikipedia removed.]
Posted by: Keenan at August 15, 2009 1:45 AMmany of the above answers are true and correct when they are brought together. yes the hole is round and yes the seal is better and yes it cannot fall into the hole. there are other factors too, such as a circle being the strongest shape therefore alowing maximum strength and being able to cope with the huge volume of weight and pressure from above the street. the portability aspect of being able to manouvre the cover due to its rondness is also true. therefore manhole covers MUST be round!!!! and no i did not miss the point of the question
Posted by: cameron at August 23, 2009 6:27 PMLeo
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 AMthe simple answer is round hole square peg.....
Posted by: john neil at November 13, 2009 6:02 PMother 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.......