Optical illusions just keep getting more extreme. We once examined an optical illusion in which a silhouetted figure spins in different directions depending on how you look at it. Here's the sequel, which is four times the optical illusion craziness.

Here's an explanation of what to look at in the video up above:

Developed by psychophysiologist Marcel de Heer, the women appear to twirl full circle at times either clockwise or counter-clockwise. At second view, they could be turning in 180 degree increments from side to side. A variant of the spinning dancer illusion, this quartet encourages many ways of seeing. What changes when you view the women in pairs or individually? Does changing your focus from the head to the feet or hands cause your perception to shift? De Heer suggests the addition of several figures allows the viewer to see many possible rotations without diverting his eyes.

You can read more about this effect over at New Scientist.