WHAT YOU NEED: ruler, scissors, pencil, piece of card. Image caption, STEP 1: Use the ruler and pencil to draw a diagonal line from the bottom left corner of the card to any point on the top edge ...

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.

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 ...