A square is a special kind of rectangle, it is one where all the sides have the same length. Thus every square is a rectangle because it is a quadrilateral with all four angles right angles. But not every rectangle is a square, to be a square its sides must have the same length.
For the same reason that a man is always a person but a person
is not always a man.
And the same reason that a 5th grader is always a student but a student
is not always a 5th grader.
A square is a special kind of rectangle. one with all equal sides. If all the
sides are not equal, then its still a rectangle, but its no longer the special
kind of rectangle.