In your own idea why is the Manhole round?

A man hole is round or geometrically circular in shape.Its always designed as a cirle becoz its the only shape which will not fall inside itself. Ex: Consider if the manholes were square in shape ,the diagnal of the square is longer than the sides,same in case for a rectangle or for any quadrangle.If a mischevious kid were to put the quadranle through the diagnal it would fall down.Since a circle wont fall inside itself any way,man holes were designed to be round.