1) Stare at the black dot for 20 seconds
2) Look at the sky (or a white wall)
3) Blink your eyes
4) You will see a rainbow there
5) Smile :)
http://www.funnyuse.com/2011/10/rainbow-illusion-stare-at-black-dot-for.html
http://themetapicture.com/rainbow-illusion/
possible