> Manholes covers are round because that is the shape that won’t fall in

It's worth mentioning that this oft-repeated canard doesn't have much evidence. Manhole covers come in a great many shapes, and this has always been true: the Romans made them square. There are plenty of shapes which won't fall in, such as any curve of constant width. Basically fake-Feynman is right: manhole covers are round because it is often convenient to make manholes round.