Most people see their hand or the wall distorting in shape.

Staring at the moving spiral tires some of your cells responsible for signaling motion. These cells are not in your eye, but in your brain.

Your brain contains separate cells for noticing movement in different directions. By looking at the balance of activity from all of these cells, you can work out which way an object is moving.

If you tire out the brain cells responsible for noticing clockwise movement, the ones that notice anti-clockwise movement are still sending signals – so this is what you see.

Next time you are sitting by a waterfall, see if staring at one part of it for a minute produces the same effect.