||You can turn a square manhole cover sideways and
drop it down the diagonal of the manhole. You cannot
drop a round manhole cover down the manhole. Therefore,
round manhole covers are safer and more practical
than square ones.
Based on "Manhole Covers,"
Lateral Thinking Puzzlesby Paul Sloane and
Des MacHale, distributed by Cassell in the UK
and by Capricorn Link in Australia.