Demos

Keep your eyes on the red dot. After the black and white dashed outlines disappear, does the entire square look gray? 

Look again, but keep moving your eyes around. There is a dark gray square that is clearly visible now.

Look at the center of the grid for a few seconds. Either the horizontal or vertical lines will fade from view. The longer you stare, the more striking the illusion.

Look at the sphere for several seconds. Do you see the sphere changing directions? Note how these reversals cannot be stopped no matter how hard you try. Now look away for a second and look back. Do so repeatedly. Amazingly, this will prevent any further changes from happening.