i know the answer but i do not know Why BUT

A Polygon simply put is a two dimensional shape formed with straight lines, a Square is a type of polygon.

A square can be considered a polygon because it matches the definition of what a polygon is. You cannot however say that a polygon is a square, because there are many shapes that classify as a polygon like triangles, rectangles, etc.

So all squares are polygons, but not all polygons are squares.

A simpler analogy is that of animals. You can say that a cat is an animal, but you cannot say that an animal is a cat, because the term animal includes a wide range of different creatures, not just cats.

If we take a step back from analogies and focus more on the programming aspect, if you have a class called Square that inherits from a class called Polygon then the Square class will have all of the properties and methods that belong to the Polygon class. That is a requirement of inheriting from it.

Because of that a Square can safely be treated as if it actually was a Polygon without it causing any issues. Treating a Polygon as if it were a Square however would not work, because a Square could have methods and properties that are not found in the Polygon class.