Holey cast-iron shapes, Batman!

Did you know that manhole covers are circular (or triangular-ish, if you live in 'Frisco) so that clumsy sanitation workers can't drop them on their colleagues?

It's true.

The reason is that the square has the annoying property that the diameter from one corner to the other is bigger than the diameter halfway along an edge. This means that if you put the square manhole [cover] on its side, it is very easy to drop the square cover down the manhole.

[Thanks to Marcus du Sautoy, The Times.]