Manholes

Conventional wisdom has it that manhole covers are round because that's the only shape that won't fall down the through the hole.

I don't think this is true, because subway grates are rectangular, and those grates found in the gutters of suburban streets are square, and they don't fall down. Just thinking it through in my mind, it wouldn't be that hard to create a ledge around the inside of a square (or triangular or hexagonal) hole to stop the cover from falling down.

I think the reason manhole covers are round is because manholes are round.

So why are manholes round?

Because they're made of sewer pipes, and sewer pipes are round.