How is a rectangle not a square?

Answer 1

a rectangle is not a square by a square having parallel sides for all but the rectangle only has one pair of parallel sides

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Actually, a rectangle has two pairs of parallel sides - the same as a square. Where they differ is that a rectangle has two pairs of sides of equal length, but each pair is different from the other. In a square, all four sides are of equal length.

Answer 2

A rectangle can be a square - a rectangle can have four sides of the same length as long as all the interior angles are equal to 90 degrees.