Shading Fades Away

Stare at the center of the black dot for about 20 seconds. After a little while, you'll notice that the outside of the circle - the shaded area - starts to lose definition, and even becomes a solid color. Or, your eyes may notice that the shading shrinks and isn't even perceptible after a minute or so.

This is quite a weird optical illusion! And it only becomes more convincing the longer you stare at the black dot, so you must be patient!