How is it that a square is a rectangle but a rectangle ins't a square?

Beacuse  A square by definition is a "plane figure having four equal sides." Rectangles' sides are not equal and hence cannot be a square. 

A rectangle by definition is a "four-sided plane figure with 4 right angles" - which also implies that a square can be a rectangle because it is also a four-sided plane figure with 4 right angles hope this helps

The definition of a square is a plane figure with four equal straight sides and four right angles. The definition of a rectangle is a plane figure with four right sides and four right angles. To be a rectangle you just need four straight sides and four right angles, but to be a square all those sides have to be equal. Therefore, a square is a rectangle because it has four straight sides and four right angles, but a rectangle isn't a square (or not always) because it doesn't have four equal straight sides.

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