This is a lateral thinking problem, so there are pottentialy many possible answers.
Question: Why are manhole covers round?
I came up with one answer, then someone told me a diffrent one that is more elegant.
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Round covers are easier to transport by hand (i.e. roll it). Almost round covers will have that property too.
Also, I've heard that a round cover cannot fall in.
Those are both correct. Anouther possible answer is that the circular shape has the most stress resistance, which could be important given that roads both exapand and shrink over the course of a year.
The answer I heard on the playground (centuries ago): . . . . . . . Because manholes are round.
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