Why are manhole covers round?

This is a very commonly asked question – asked in interviews with Microsoft, Google, and many other companies.

Although this question really has nothing to do with any programming job, it gives an interviewer the opportunity to see how a potential candidate would structure their thinking when approaching a problem.

Break the Question Down




Keep in mind that getting the right answer to this question is not as important as showing the interviewer that your thinking is structured logically. When approaching a question like this, it helps to get to a more specific question and then try to answer that question instead of the one originally asked. It’s like someone were to give you a 3 week project, consisting of many different tasks – you basically divide the project into tasks to get the whole thing completed. Similarly, you want to "break" the question down into more manageable "pieces", because you can’t really answer it the way it is.

The Many Answers to the Question

So, you may make the question more specific by asking yourself how would the fact that a manhole is round make someone’s (like someone who works in the sewers) job easier? That might get you thinking about what someone like that would do on a day to day basis – like lifting the manhole cover and putting it back in its place.

Now that sounds promising – how would the fact that a manhole cover is round make it easier to put its back in its place? Well, for one thing with a circular shape you don’t have to adjust the cover to fit the hole – it will always fit. But, let’s say the manhole cover were a different shape – like a square.

Would a square manhole cover fit a square shaped hole no matter how the cover is aligned? If you think about it, the answer is clearly no – you will have to adjust a square shaped cover to fit a square shaped hole. Since that’s not true for a circular shaped manhole cover, that’s obviously one advantage to having a circular manhole cover.

Mishandling the Manhole cover




What else could potentially happen with a manhole cover when someone is handling it? What if someone accidentally dropped the cover on its edge? Well, if it’s a circular shaped manhole cover then it will not fall into the hole because the diameter of the manhole cover is equal to the diameter of the hole. What if the cover and hole were not circular in shape – like a square, for instance. Well, is there any part of the square shaped hole where a square shaped cover could fall in?

Let’s make that question more specific: what’s the longest part inside a square? Well, the diagonal is the longest part. The pythagorean theorem (remember that?) will tell you just how long: multiply the length of the side of a square by the square root of 2 and that will give you the length of the diagonal. Well, that means that any side of the square can fall into the hole if held at the right angle. This is the problem with a square shaped manhole cover, and the advantage of a circular shaped manhole cover.

Rolling the Manhole Cover

What about carrying the manhole cover? One last advantage of the manhole cover being round is that you can roll it around. That’s not possible with any other shape.

Round Manhole Covers don’t have corners that need to be lined up

Another advantage of a round manhole cover is the fact that when placing the manhole cover on top of the hole, you don’t need to adjust the cover to fit the hole. However, if you were dealing with a triangular manhole cover, you would obviously need to adjust the cover until the corners lined up with the hole as well.

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