ExplanationMost people think this is some sort of trick, with the animation gradually changing in front of your eyes. Not so!
The illusion relies once again on persistence of vision. The human eye forms an image on the retina - the light sensitive part - and transfers information about the image to the brain. Just like film in a camera, this process involves reactions. The light falling on the retina stimulates nerves. It takes time for the stimulation to clear. Normally we don't look at one thing for a long period, so further stimulation happens, over-riding the last image about 20 times a second.
If we stare at things, however, things are different - especially when what we stare at contrasting colours with a solid background colour. The eye grows used to the image it sees (in this case, mostly grey), and the nerves go on relaying the same information, over and over.