Why are manhole covers round? Yes I know...because the holes are round. But why round? Anyone?

Because a round cover can't fall through the round hole regardless of which way you position it. If you have a square or rectangular cover, it can fall through the hole - just stand it on it's end and turn it diagonally. The length of one side is shorter than corner to opposite corner of the same sized square or rectangle.

I'd imagine this wouldn't be pleasant, and was likely a problem at one time to make them change their style to round. Secondly, manhole covers are HEAVY - I believe they're made of cast iron, or something similar. A round manhole cover can be rolled by one person if it needs to be moved - other shapes may require more than one person to carry.

They're easier to line up as they fit in any way you turn them It's easier to dig a round role or create a round pipe than it is to dig a square hole. A round shape requires less material (volume) to fill the same dimensions than any other shape - mathematically for the same reason that bubbles are spherical. The typical "correct" answer is the first, but any others will also work well.

This is always an interview question supposed to test "critical thinking" - most people know the FIRST answer either from preparing for interviews or from being told in an interview (which is how I learned). If you are trying to impress an interviewer, use some of the other answers - such as the fact the round shape is easier to move and requires less materials. This shows you are thinking, and it's less likely to be from straight memorization because it's not the common classic answer - but it's still right!

Good luck!

Here's some trivia for you, a Reuleaux triangle is another manhole cover shape that couldn't fall down the hole.

Weird. The obvious answer to me would be because drills are round, and they dig man holes with drills. Huh.

I know this question already has a best answer but manhole covers are round to cover a round hole which is basically the shape of a person going down it. Additionally the reason for the hole being round is because this shape is the strongest against collapse by the pipe. If it was because of falling in then storm drains wouldn't be rectanular.

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