Question

why are manholes cover round??

Answer 1

Because if they were square the covers can fall inside diagonally

Answer 2

Manhole covers are round because a circle is the only shape that won't fall upon itself. It's like if you're having a triangle with 2 sides equal and one unequal side, which maybe longer or shorter than the other two sides, there's a chance that you might forget how you put the manhole cover in the first place. Further, however you rotate it, the circlular cover won't fall into the hole, which can be the case for covers of other shapes. That makes it more convenient to have round manhole covers.