Why are manhole covers round?

The most common answer is simply that a round manhole cover can't fall into the hole it covers. But I did tell you that there's more than one answer. Heavy, round manhole covers are easier for one person to move by standing it on end and rolling. Also, you don't have to "align" a manhole cover to it's hole. Any orientation will do.

My father, a mechanical engineer, claimed an answer I hadn't heard before: that round manhole covers are easier to manufacture. That's a nifty answer and it would have done great in the interview except he also claimed that was the only reason and all the others were wrong. Whoops.

And for the record, round is not the ONLY shape that can't fall into its hole. There's a class of such shapes referred to as "Reuleaux polygons". The common characteristic is that they share a constant width or diameter.