Optical illusion: impossible anti-grav ramps

Koukichi Sugihara's "Impossible motion" -- a cunning arrangement of cardboard ramps carefully skewed to create the illusion that balls roll up them -- won top honors at the Meiji Institute for Advanced Study of Mathematical Sciences' Best Visual Illusion of the Year Contest 2010. It has a delightful 3D physicality, being a real object made out of atoms, that makes it especially wonderful.

Impossible motion: magnet-like slopes (via Neatorama)