Manhole covers are round
A square or rectangular lid would be easier to make and fit. Yet nearly every manhole cover is a circle.
Category: Streets & signs
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Feature. A circle can never fall through its own hole.
A round cover is the same width in every direction, so it cannot be turned and dropped down the hole it covers. It is also easy to roll and does not need to be lined up with the opening.
Source: Manhole cover, Wikipedia.
