The saying "you can't believe your eyes" reflects a relatively new idea. In the early 1800s everyone believed their eyes. Not because people were gullible, but because their eyes were the most accurate and effective tools they had to study the world. So it was natural for scientists and others to be curious about how the eye itself worked. Optical illusions were particularly interesting because they seemed to show that the eye could be tricked.
The fanciful toys you'll learn to make on the following pages all began as serious tools scientists used to study one such trick: the illusion of apparent motion. You experience apparent motion whenever you watch a movie. A film projector actually shows only a series of still pictures very quickly, one after another. Your brain pieces them all together to look like natural movement.
How It Works: All animation, from flip books to movies, works because of an illusion called apparent motion. In the flip book above, all you really see is card after card showing a little house and a car. In one picture the car is here and in the next it is there, though it never actually moves from one spot to the other. But view picture after picture rapidly, by flipping the pages of the book, and somehow the different pictures blend, giving you the impression that the little vehicle is scooting along the road. Scientists say your brain fills in the gaps to create smooth motion because it is trying to make sense of what you are seeing. You don't even need an optical toy to experience this phenomenon; it happens constantly-every time you blink. The reason you don't notice blinking as a black screen is that your brain fills in the brief dark period with a reasonable assumption about what happened when your eyes were closed. Similarly, when you look at an image of an object, a split second of blankness, then an image of the same object in a new spot, your brain fills in the gap, and you "see" the object move.