Split the parallelogram into a right-angled triangle and a trapezium. Move the triangle to the other end of the shape, making a rectangle. The length of the rectangle is the base of the parallelogram.

Move the shape of the peaks to create different quads . Is it possible to make a square that is not also a rectangles ? A diamond that is not also a parallelogram ? Why? Drag the vertices to create ...

The rectangle is a wobbly, unstable shape. When you push the side, it flops into a slanted parallelogram. This happens without any of the rectangle's sides changing length. When you poke the top ...