[Gravdigr]

from games.yahoo.com

Which of the two colored shapes in this illusion is rotating? If you guessed ‘both,’ guess again. Only the outer red ring is actually moving; the blue inner core is stationary. But every time the star-shaped projections on the two objects line up perfectly, the blue shape appears to ‘jerk’ backwards, or counter-clockwise, almost like a gear slotting into place. What causes the illusion? Dutch mathematician and cognitive psychologist Dr. Peter A. van der Helm writes: “[T]he rhombi jump backwards from one wheel notch to the next, and we think the visual system mistakes the wheel notch that opens a background area for the one that shut this background area a moment before. This mistake occurs especially when the wheel has a lower background contrast than the stationary shapes…” In layman’s terms, the brain gets confused by the growing and shrinking gaps between the two shapes, and the strength of the illusion depends on the contrast between their colors. Simple, right? Well, maybe not. But it is nifty. See more about the illusion and its variants here: http://bit.ly/GTRICK

If you scroll down to where you can see only the bottom three points of the blue star, the effect disappears.