I just saw this at newscientist.com.
Here's the explanation:
If you follow the movement of the rotating pink dot, it remains pink. However if you stare at the black cross, the moving dot appears green. If you concentrate on the black cross for a while, all the pink dots will slowly disappear, leaving only a rotating green dot.
There is no green dot, and the pink dots don't disappear. How does this work?
(If the dot is not moving, try hitting the refresh button, or go to
this site.)