Solution #95: Cover That Hole

A square or rectangular manhole cover can fall down the hole, while a round manhole cover cannot. The square cover will fit down the diagonal of the hole (unless the rim it sits on is very large) but no matter how you turn a circle it never measures less than its diameter. So for safety and practicality all manhole covers should be round.

Alex Freuman also notes: "In addition to the advantage that a circular manhole cover cannot fall into a manhole, one can roll a circular manhole cover rather than lifting it."

Source: Sloane, Paul and Des MacHale
Categories: Clever, Geometry and Space, Favorite
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