To demonstrate an optical illusion that occurs due to motion of objects.
Take a pencil and hold it between the thumb and first finger at one end as shown in the diagram. Then shake the hand straight up and down in short, quick motion a few inches. Don't move the pencil with your thumb and finger but hold it in a slack grip so that it wiggles slightly as the hand goes up and down. The pencil will appear as though it were made of rubber that bends with every movement of the hand. This is due to a sensory optical illusion.