Optical illusion: Stepping feet

The stepping feet illusion, as discovered by Anstis (2003) Neural Netw 16. When the background texture is visible, the two 'feet' appear to be stepping out of sync. While in reality, of course, they are not. The illusion is strongest for some people if you don't look at the feet directly. For an explanation, see http://www.cogsci.nl/illusions/stepping-feet-and-tipsy-toddlers