Why is a manhole cover round?

It is impossible to drop a round lid into a smaller round hole.

It is because round is the only shape that if turned or flipped over, it still retains the same length across its center. If you used a square, rectangle, triangle, hexagon or any other shaped lid, it could be rotated and fall into the hole.

This can't happen with the round shape.

And I have worked for the phone company for the last 22 years, yes it is a pain to get a rectangular lid that weighs 100 lbs, out of a 6 foot deep hole by yourself.

Two more correct answers

A round cover can be moved by rolling it on its edge.

A round cover contains less metal than a same-sized cover of any other shape.

actually a round manhole cover isint the only shape that would work a reulux triangle would work too and square manholes would puncture tires