Do you see a ball moving back and forth at the top and below this moving ball do you see two stationary balls?
Would it surprise you if I said that the moving ball is really one ball presented first on the left over the stationary ball and then on the right over the right stationary ball? This is true even though it appears like one ball moving across the screen.
This is an old and famous illusion called the Phi Phenomenon. If you think about it, you've seen this principle many times on movie marquees. The principle behind this phenomenon also lies behind the moving pictures you see at the cinema. Did you realize that at the movies you are looking at a rapid sequence of discrete images?
The two balls do not necessarily have to be the same color. Click on Color Phi to see what I mean.
Click on the following link to see a proposed distinction between phi and beta movement.