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. Full colour optical illusions Hours of fun trying to spot the dual images Previously unpublished works by Escher and Dali.