First you see one thing, then another. The picture changes from one moment to the next, flip-flopping back and forth. That's an ambiguous illusion, and the ones in this largest-ever collection offer constant, pleasurable surprises. Sometimes, the perceptual shift occurs if you just stare at the visual puzzle long enough; in other cases, it's necessary to invert or rotate the image first. In one picture, it's a cat-and-mouse game as you try to discover where the mouse is hiding from the cat. Another image features two minstrels playing homage to William Shakespeare - but at first, only the familiar portrait of the bard may be apparent. Once the dual nature of the illusion reveals itself, you'll never see just one object again.