That's false. Not all parallelograms are rectangles, only some of them are. If a parallelogram wants to be a rectangle, then it has to agree to have a right angle (90 degrees) at one corner.
(I know, I know. Rectangles have right angles at ALL four corners. But if a parallelogram has one right angle AND it's still a parallelogram, then it turns out that all four of its angles are right angles. So if you tell a parallelogram that one of its angles has to be a right angle, and it wants to still be a parallelogram, then it must turn into a rectangle.)
