Give reasons for the following: Square is also a parallelogram.
Hint: Square has all sides equal and opposite sides parallel to each other.
In a square, all sides are equal, alternate sides of a square are at right angled i.e. perpendicular to each other and opposite sides parallel. In a parallelogram, all sides are equal and opposite sides parallel. So, we conclude that a parallelogram cannot always be called a square as there is no stringent rule about the angle between two alternate sides. But if that angle becomes equal to, then it becomes a square. Hence, square is an example of a parallelogram when the angle between alternate sides becomes $90^0$.
Note: For this type of problem, always try to keep in mind the constraints required. Here we saw the constraints to be a square and a parallelogram. Thus, on comparing both, we figured out the reason.