Why are most manhole covers round?

Why are most manhole covers round?Sure, it makes them easy to roll and slide into place in any alignment but there's another more compelling reason involving a peculiar geometric property of circles and other shapes.Imagine a square separating two parallel lines.As it rotates, the lines first push apart,then come back together.But try this with a circle and the lines stay exactly the same distance apart,the diameter of the circle.