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Why Are Manhole Covers Round?

August 29, 2016

It’s supposed to be one of the hardest interview questions. I think it’s been asked so often that it’s become a cliche. When I worked for Microsoft, it was common to ask oddball questions. Some of the ones that were used:

  • How many gas stations in the United States?
  • How do they get the coating on M&M candies?

And, of course, the manhole question.

The point of the questions is to figure out how well the candidate thinks on his feet. The interviewer is less concerned with the actual answer than they are with the thought that went into it. Unfortunately, for me, I look at them as real questions with real answers.

The manhole cover was my favorite. There are four reasons that I’ve heard of for why manhole covers are round.

1. They are round because the circle is the most efficient form. You get the most usable space with a circle. It’s also the strongest geometric form. Your circle is going to provide equal support against the manhole collapsing. 

2. They are easier to move. A manhole cover can weigh between 90 and 150 lbs according to Government Security News. That’s a heavy, not to mention bulky object to attempt to move. But, if it’s round, you can move it simply by tipping it on its edge and rolling it. 

3. A round manhole cover cannot fall into the manhole. It’s physically impossible for a manhole cover to fall into the manhole if it’s round. And at potentially 150 lbs, getting it back out of a hole would be challenging. 

However, my favorite answer has nothing to do with geometry or physics. 

4. They are round because the makers decided to make them round. 

Of course, I haven’t been asked this question in years. But, if it comes up in conversation, I’m ready. 

Rodney M Bliss is an author, columnist and IT Consultant. His blog updates every weekday at 7:00 AM Mountain Time. He lives in Pleasant Grove, UT with his lovely wife, thirteen children and grandchildren. 

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