This works on the optic nerve of the eye. I remember reading a Time/Life Science book from 1969 that show an American flag with yellow, lite green and black stars. The idea was to stare at it for a period of time than look at a sheet of white paper. You would see the image of the flag on the paper but only this time it was in the red, white and blue colors. Fooled the brain again! Its neat how the mind works.
I recently saw the same thing on some web site, except the flag was a Union Jack.
Notice the circle with the pink dots, if you look away from them even after looking at it for only a second or so, you see a circular pattern of green dots. So if the pink dots disappear even briefly, you will see the green image.
I looked at the black X for up to a couple of minutes, but the pink dots never went away.
Here's another one...
Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht
oredr the ltteers in a wrod are; the olny iprmoetnt tihng is taht the frist
and lsat ltteer be at the rghit pclae. The rset can be a total mses and you
can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deos
not raed ervey lteter by istlef, but the wrod as a wlohe.
Amzanig, eh?
No wonder it's so hard to proofread and not still miss a typo.