Question

How is square a rectangle?

Answer 1

A rectangle is a parallelogram whose sides intersect at ${90}^{\circ }$ angles.
A square is a specialized case of the rectangle.
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.