Explain why a square is always a rectangle but a rectangle is not always a square.

A rectangle is a plane figure with four straight sides and four right angles. A square is a plane figure with four equal straight sides and four right angles. 

therefore, because a square has 4 straight sides with 4 right angles, it is a rectangle. 

and a rectangle can have unequal adjacent sides, so it is not always a square

Do the inverse operation