Why is a manhole cover round? A Microsoft interview question.

OK, so it probably isn't an "official" interview question (if there are any), but I have asked it during interviews in the past. For me, it is simply a way to help loosen up the interviewer and have some fun, but if the only answer I get is "I don't know, why is it?", then that gives me the impression that the person either isn't trying to really interview, or that they just aren't very creative.

So, why is a manhole cover round? Heck if I know, but here are some good answers that I've heard over the years:

1. Because the hole is round (duh!)
2. Because animals dig round holes, so it feels natural to humans too
3. Because a circle offsets the straight lines of a city
4. Because it is easier to roll the cover some distance than carry it
5. Because it won't fall into the hole - but, the same is true for an equilateral triangle
6. Because it is easier to pour hot metal into a circular mold than one with sharp corners

I suppose there are more, and there is probably some web site with the "correct" answer. But I like these because I can remember the individuals who have told them to me over the years. What's your answer?

Comments

• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
  Cool answer Leon! Now if we combine the ideas of an ever shrinking concept of "now" (what is the smallest unit of time anyway?) with an ever reducing circumference, can we move subatomic particles at light speed through our manhole?
• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
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• Anonymous
  September 27, 2004
  This is the answer, I would give.
  
  When you heat metals, they expand. Assume that you choose any shape other than circle, when they expand you can not know which parts becomes longer than other parts. But with circles they expand (almost) uniformly. So even with expanding they are easier to remove and put back in. I mean when they expand they do not get stuck.
  
• Anonymous
  September 27, 2004
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• Anonymous
  September 28, 2004
  Here is my answer:
  
  Microsoft secretly bought the company that makes all the manhole covers and, even though round is not the most efficient design, Microsoft controls the industry and will keep it round just because they think its the most efficient and most profitable.
• Anonymous
  September 28, 2004
  And this is why I'll never get a job at Microsoft. I don't imagine you've had anyone reply with. "Yeah... I don't answer stupid questions. I'm here to code, not play trivial pursuit."
  
  
• Anonymous
  September 28, 2004
  GUYS.
  
  Just read this. An answer to the age old question is given here.
  
  http://ask-leo.com/so_just_why_are_manhole_covers_round.html
  
  It also links to http://www.maa.org/mathland/mathland_10_21.html which are "Reuleaux polygons" (constant width or diameter).
  
  END OF DISCUSSION! :-)
• Anonymous
  September 28, 2004
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• Anonymous
  September 28, 2004
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• Anonymous
  September 29, 2004
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• Anonymous
  October 13, 2004
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• Anonymous
  May 04, 2007
  PingBack from http://andywine.com/blog/2007/05/05/creepy/
• Anonymous
  May 09, 2007
  PingBack from http://jakesprouse.net/code/?p=6
• Anonymous
  October 21, 2008
  A couple of years ago I wrote up “ Why is a manhole cover round? ” and got a few good comments on it.