Quick
Answer:
A manhole cover rests on a small lip inside the hole.
A circular manhole cover
typically will not fall into
the hole because its width is the same all
around.
A rectangular manhole cover,
however, could fall through the
hole when it is tipped upward. You can see
this by drawing a diagonal in a rectangle to create two
triangles:


Complete
Solution:
Mathematically, the greatest
angle of a triangle is opposite the longest side.
The greatest angle in each triangle formed by drawing
in the diagonal is the right angle at the corner.
This means that the diagonal
of a rectangle is always longer than either of the sides.
As a result, rectangular
covers can always
be dropped through their corresponding holes
if the lips are small. Because a square is a rectangle,
the same reasoning applies to squares. 
