Possibly the greatest optical illusion ever http://www.slate.com/blogs/bad_astronomy/2014/08/05/illusion_rotating_rings_change_directions_when_you_look_away.html There's an entire neurobiology PhD thesis on it! https://digital.lib.washington.edu/researchworks/bitstream/handle/1773/25444/Meilstrup_washington_0250E_12882.pdf