In this activity, students take advantage of persistence of vision to combine two separate images.
A thaumatrope is a toy, popular in Victorian times, that uses this persistence of vision.
This illusion takes advantage of something called "persistence of vision".
When an image is shown to your eyes, the retina keeps responding for a short time (about 1/30th of a second) after the image itself has gone away. If a new picture appears just before the previous one has faded away, your brain blends the two together.