This is an optical illusion called "stepping blocks". After starting the project, click "Go" to make the blocks go. The idea is that if you look roughly in between the yellow and blue blocks, it looks like they are taking turns getting in front of the other -like they are walking feet. But they aren't. Of course, they are actually going exactly the same speed. It works best if you don't exactly stare at the blocks. Use the controls to start and stop the blocks, and you can see that they are always going exactly the same speed. If you change the contrast, you can also make the illusion go away. Thanks to @Except for the idea of having a solid black background as a contrast layer. Leave comments about what you think. Does the illusion work for you? The code is pretty simple, so you can try lots of variations.
Thanks to the Michael Bach page of optical illusions. http://www.michaelbach.de/ot/
