Re:Manhole covers (Score 1)

the round shape prevents them from slipping down the whole if someone sets it down at an incorrect angle.

OK, this has been bugging me for awhile. If falling down the hole is the only concern, then the cover could be any shape, as long as the smallest dimension was still too big to fit through the hole. So it seems that there's an additional unstated assumption: the cover must be as small as possible. Anything else?