Question

How can we say that a square is a rectangle but a rectangle is may not be a square?

Answer 1

$\Rightarrow$ A rectangle is a quadrilateral with all four angles right angles and opposite sides are equal.

$\Rightarrow$ A square is a quadrilateral with all four angles right angles and all four sides of the same length.

$\Rightarrow$ So 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. However not every rectangle is a square, to be a square its sides must have the same length.