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Comment by Silhouette | Hacker News Reader

The interviews had ridiculous questions like “why is a manhole cover round?”

That was always one of the worst standard brain-teaser questions because so many people who asked it didn't know the correct answer.

Hint: If your answer was that a circular cover is the only shape that can't fall down the matching hole, everything you think you know about the world is wrong and the term you're looking for is "curve of constant width".

The real answer is because manhole covers are mass produced and manufacturing circular ones is usually easier and cheaper than the alternatives.

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Manhole covers are round because manholes are round and manholes are round because pipes are round. Even in America there are plenty of square grates and covers on the ground, it's just that they're not covering pipes.

The pipes used are also mass produced and manufacturing circular ones is also usually easier and cheaper than cross-sections with other shapes. Obviously there are other important considerations like the ease of tunnelling and the strength of the pipes when buried underground as well.

However, one thing that none of this depends on is a circular cover being the only shape that can't fall down the corresponding circular hole. That was the standard answer that way too many companies that used this particular interview question wanted and it was wrong.

The real answer is they aren't - they are sometimes round.

https://duckduckgo.com/?q=rectangular+manhole+cover&t=brave&...

https://duckduckgo.com/?q=triangular+manhole+cover&t=brave&i...

https://duckduckgo.com/?q=hexagonal+manhole+cover&t=brave&ia...

https://duckduckgo.com/?q=oval+manhole+cover&t=brave&iar=ima...

And so on... Now a great - though a bit unfair - question would be to ask about circular ones, listen to the standard pitch, and then show these photos and ask - so what's going on here? Are all these people idiots?

A good software engineer knows there are rules, and there are reasons for the rules, and there are reasons why sometimes rules need to be different because the situation is different. If you just learn one single "right" answer, you'd end up trying to fit a round manhole cover into a triangular opening.

> The real answer is because manhole covers are mass produced and manufacturing circular ones is usually easier and cheaper than the alternatives.

My understanding is the primary reason is because circular covers cannot fall into the hole itself, whereas that problem exists with other shapes like squares.

The parent comment already addressed this.

Shapes of constant width (e.g. £1 coin) won’t fall into the hole either. Neither would an equilateral triangle.

Oh, and by the way in my country we have square-shaped manhole covers too :O

Equilateral triangles can fall in. You have three dimensions: the height of the triangle is less than the side length of the triangle, so it fits in near the sides of the hole.

The OP mentions curves of constant width:

https://en.wikipedia.org/wiki/Curve_of_constant_width

Which includes circles and Reuleaux triangles, which are much more difficult to manufacture than circles. I think this can be rounded off to "circles are the only well-known shape that can't fall into a similarly-sized hole".

You’re right, I didn’t think through the triangle.

Still, as I’ve said, there are square-shaped manhole covers too.

http://manhole.co.il

Would a square-shaped manhole fall into a circular cover?

Also in some countries manholes are not always round but also square (hinged to the road to avoid falling).

Is it easier and cheaper to manufacture because a circle has no obvious weaknesses? For example most point shapes are weak structurally weak at the corners.

There are infinite shapes that cannot fall down the matching hole, but the circle is the easiest to manufacture. A circular cover is easier to manufacture than e.g. a Reuleaux triangle shaped one.

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