I love this one. Which direction is our gal spinning: clockwise or counter-clockwise?
Look closely. You’ll initially see her on her left foot spinning clockwise, or on her right foot spinning counter-clockwise. Look again. Try to mentally change the direction. Ride that line and see if you can control what you see. If you find it difficult to reverse the direction of her spin, concentrate on switching her feet.
Upon careful observation of her shadow, you’ll see that she is in fact on her left foot, spinning clockwise.
For a frame-by-frame analysis, check out this.
For a feline twist on this classic, check out the Spinning Cat Illusion.