Tip a square cover up on its edge and it drops through the hole along the diagonal. A circle is the same width whichever way you turn it, so it can’t. You can also roll a round one, and it doesn’t need lining up to go back on.
Why is a round manhole cover better than a square one?
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Think about the diagonal of a square.
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A round cover can’t fall down its own hole
Source: Anonymous, 20th century