A rotating figure 8 made of two overlapping rings is ambiguous. Small spots painted on the rings can resolve the ambiguity, forcing them to look like an 8 or like 2 rings. Even without any spots, if the brightness of the intersections where the rings overlap makes the rings look transparent, they slide. If not, they stick.
UC San Diego, USA, Université Paris Descartes, France
(© 2011 Stuart Anstis & Patrick Cavanagh)
VN:F [1.9.22_1171]Illusions from rotating rings,