Why are manhole covers round?
Why are manholes (and their covers) round?
If a manhole is square, or triangular, or hexagonal, or elliptical, or any convex shape other than round, it's possible for the cover to fall down the manhole. With a round manhole, it isn't.
A good, simple, metapuzzle.
This is one of several pages on puzzles and metapuzzles .