Wrap your visual cortex around this:
Notice that even though this is not a .gif, the part of the image on the peripherals of wherever you are focusing seem to move. You can prove this to yourself by focusing past the image. Once you do so, the image stops moving.
Optical illusions are failures of your brain software. Your brain has 3D models of textures and shapes that describe your immediate reality. When certain patterns confuse this software, it seems to baffle us.
Still cool though.