A thaumatrope is when you spin two images quickly to simulate motion. The images seem to be in motion because of persistence of vision. Persistence of vision occurs because an image stays in your head a fraction of a second after it is gone from your eyes. Since the next image comes before the first one can go away from your brain, you see a consistent motion. Persistence of vision is why animation works. It is a very important factor in creating any kind of animation.
Down below is an example of a thaumatrope
Down below is an example of a thaumatrope
This is a good example of a thaumatrope because the images on both pages look like they are together. The placement of the image on the second page is at the perfect spot. If the image was lower, it would look weird because it would look like the jet is moving around his eyes and you would be able to see the eye through the line. If the image was too high it would look like a halo instead and ruin the picture.