optical illusions are fun -- I like messing with the ol' noggin'...
stare at the "+" in the middle soon you'll see a green "dot" magically appear 8)
(http://img371.imageshack.us/img371/351/illusiondot1ks.gif) (http://gwally.com/avatars/000538.php)
(found it on this insane page of animated GIFs (http://gwally.com/avatars/cat_animated.php))
That is really cool... Interesting that it's the switch from pink back to grey that makes the green appear.
What's really cool is that it only takes a second or two (many similar illusions you have to really stare a long time) and BONUS, if you look to the side AFTER seeing Mr. Green Dot, you'll see that all the pink dot have been burnt "green" into your retinas :) (i.e. you'll see an echo image of the outer circle of dots, but green instead of pink).
Messing with sensory perception: free, harmless fun :D