Figure 1. Rotating ring illusion.

(A) A Styrofoam ring is seen at an angle to another ring as wobbling or rolling over the bottom ring despite physical implausibility. (B) At a slower speed, the rings are seen to rotate together revealing that they are glued together, and that the non-rigid rolling was an illusion. (C) Two rings rotate together with a fixed connection at the red segment. (D) Two rings wobble against each other shown by the connection shifting to other colors. (E) & (F) are the same as C & D except that the colored segments have been painted black. Both pairs generate identical sequence of retinal images so that wobbling and rotation are indistinguishable.