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Figure This!

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:

Rectangle and Diagonal


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.

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