Here we have an animated gif file. If you look closely you can count 12 little people. After the gif slides the image around you end up with 13 little people. How does this happen? I saw the same optical illusion done once with solid colored rectangles, the rectangle one is easy to see what happens, but this one is much more difficult because it is people.
If your brain starts to seize up and you must have an answer then click on the picture and it will take you to laurenceholbrook.com where they devote a lot of words and images to making this possible to understand.