In this video, Inside Science explains how we see certain illusions as shifting pictures. Our brain takes a frozen image and begins to fixate on a certain area of the photo. We do this multiple times but when we fixate on one spot for a few milliseconds, we move onto the next area of the photo while our eyes are continuing to remember the last part of the photo that you saw, which is what gives the illusion of movement. Neuroscientists are still not exactly sure what causes our brains to function this way.