Why manhole covers are round

There is a stupid manhole cover interview question and it annoys me. A circle can't fall through its own hole no matter how you tilt it, a square one rotated onto its diagonal just slips through and drops on whoever's underneath, same deal with a rectangle or an oval, anything with a "long way across." Circle's the same width in every direction so there's no angle that lets it through. Makes sense I suppose BUT it's barely the real reason, which irked me a bit once I found that out because it's such a satisfying answer.

If you asked the folks who actually make and install these things, you get duller answers that are not the answers the interviewer is after. Round covers don't need lining up, you just drop it on and it fits no matter which way it's facing. A square one has to line up with the frame exactly, which sounds trivial until you're the poor sod wrestling a hundred-plus pounds of cast iron into place in the rain.

Also, lowkey the best bit, you don't carry a manhole cover. You roll it. On its edge (like a coin). Try that with a square one, and it's probs not gonna work.

There's a structural argument too and it's honestly the one engineers reach for first. A circular cover spreads the weight of everything rolling over it evenly round the edge, cars, trucks, the odd bin lorry. Corners are where stress builds up, so no corners means no weak points, which means less material handles the same load, cheaper to cast and it doesn't crack as easily.

So it's not one clean answer, it's a pile of separate ones that all happen to agree with each other. The shape stops it falling through. The same shape makes it easy to move. And it turns out to be the strongest option structurally too. None of them needed the others to be true, they just all kinda agree with eachother, No idea why nobody tried a triangle instead.

The interview question survives because "it can't fall through" is such a satisfying thing to land on out loud. The real answer's a lot less exciting, just a load of boring uncoordinated reasons stacking up until it looks inevitable after the fact.

Which is also probably why nobody at the actual foundry, casting the things, has ever once stopped to ask why they're round.