Optical illusion: Relative motion (Super Mario)

In this illusion, Mario appears to move vertically, whereas the shell appears to move horizontally. In reality, as you can see when the background stops moving, both Mario and the shell move in an identical counter-clockwise circle. For an explanation, see http://www.cogsci.nl/illusions/relative-motion-in-super-mario-land

Based on Stuart Anstis' "Flying Bugs" illusion.
Figure credits: mario.wikia.com/
Wallpaper credits: ipad-wallpapers.com/displayimage.php?pos=-1177