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 ...

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 ...