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.

Flip Book How-To
Our flip books are stacks of 20 or more index cards held together with bulldog clips. Illustrate the cards using rubber stamps, stickers, or cut-out paper. The objects that are supposed to "move" should be shifted by 1/8 to 3/8 inch from card to card. If it looks too jumpy, make additional cards whose images show smaller changes, and insert them in between every other page. To put all the pages in the right order for viewing, the book should begin with the last page: You flip from back to front.

Thaumatrope How-To
The thaumatrope starts as three pieces: two disks, each with a simple image, and a 16-inch-long, 3/16-inch dowel for a handle. Make color photocopies of the bird and cage or fish and bowl pictures on card stock, enlarging them to be about 4 inches across, and cut out. If you'd rather make your own, create a simple pair of related images on two card-stock disks that line up when held back to back. Glue the dowel to the back of one disk so that it runs right down the center of the image; let dry. Then glue on the second disk.

Phenakistiscope How-to
You will need black card stock, glue, scissors, a tack, and a pencil with an eraser. Photocopy the disk in color, enlarging it to about 8 inches across. Glue it to the card stock; then cut it out, cutting the 12 slots. Push the tack through the center and into the side of the pencil eraser. To make your own pattern, trace the enlarged slotted disk onto white paper, and use stickers and markers to create a sequence of 12 images in which the starting image is also the end; each image should fit within a pie-piece shape and "move" about 1/8 inch from the one before. Glue the patterned disk onto black card stock, cut out, and tack to the eraser.