A parallelogram is a 4-sided shape where opposites sides are parallel. A rectangle is a special case of a parallelogram. All rectangles are parallelograms.
But a rectangle is a shape where opposites sides are parallel *and* all the corners are 90 degree angles. So you can't say that all parallelograms would be rectangles. Some parallelograms would be rectangles, but not all.
Rectangles are a subset of the shapes called parallelograms. But parallelograms are *not* a subset of the shapes called rectangles.
It's similar to saying all cars are vehicles. But you can't say that all vehicles are cars.
