Which shapes are moving, and which are stationary?
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.