why are manhole covers round? does the square manhole cover really will fall if its put diagonally?

Question

This is an interview question and its answers say that if the cover is a square then it will fall inside when put diagonally? But how will it fall if the filling area is a smaller square? Is the filling area not small then how is it made

Answer 1

Of course a square manhole cover will fall through the square opening if you hold the plate vertically and rotate it $45^\circ$ about the vertical. The problem has nothing whatsoever to do with area.
Does this help?

By the way, look up Reuleaux triangle!