Why are manhole covers round rather than square?

This Question was most interviewer's favourite question for years. I hardly find anyone asking it these days. Answer to this question involves some simple maths with commonsense.
All of us know that circle has same diameter in all directions so no matter how you hold it cover cannot be pushed into a circular manhole. Whereas, if it is square, cover can be pushed into manhole via diameter by holding it on edge. Another aspect is, if cover is circular it is easier to roll it so for shorter distances transportation is easy.