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