The Spinning Dancer and the Brain

• This image, originally created by Nobuyuki Kayahara.
• Some observers initially see the figure as spinning clockwise and some counter-clockwise.
• The illusion derives from the lack of visual cues for depth. For instance, her arms could be swinging either in front of her to the left or behind her to the left, and hence with her circling clockwise or counter-clockwise on either her left or right foot. 
• She changes leg because she is facing either towards or away from the observer, there being no surface features on the silhouette to indicate at any point which side of her is presented: the least ambiguous positions are her profiles when she is on either side of her circle, though it's still not known whether the foreground or background leg is on the floor, and from where she moves indeterminately either on the near or far arc across to the other profile.

See also left brain vs. right brain Test by Herald Sun