A square is also a rectangle because it possesses all the properties of a rectangle. Similar to a rectangle, a square has: interior angles which measure 90° each, opposite sides that are parallel and equal, and many more attributes as well. Keep in mind however that a square is a special type of rectangle because it posseses some additional properties which don't apply to normal rectangles like the fact that all four sides of a square are equal, and diagonals of a square bisect each other at right angles.
I hope this helps! <3
