Mike sent this visual illusion. It was devised by Jeremy Hinton some time before 2005. Stare at the +sign. Three effects will appear in order: First, the gap may appear to move clockwise, or a single disk adjacent to the gap may appear to move anticlockwise. This quickly stabilizes to become a gap moving clockwise. Second, the moving gap appears as a green disk. Third, the moving green disk will appear to wipe out the lilac disks, until only the green disk is visible. Here’s a complete explanation along with an interactive version of the illusion.