Human perception is a fascinating thing to fool around with. There are numerous illusions which make you see things that are not really there (without using hallucinogenic drugs).
One of the most intriguing optical illusions is the colourful black and white image.
Just stare at the dot in the picture below for 15-30 seconds (the longer the better). Then move with your mouse over the image, but keep focusing the dot. You will see a b/w image, but it appears like a colour photo. If you move your focus from the dot, the illusion collapses.
You can easily create this illusion yourself. John Sadowski posted a tutorial for making the two images from any colour image. He even included a Photoshop action which automates the process (english Photoshop only, use my adapted action if you have a german photoshop)
You can then use my simple roll-over-generator to create the rollover-effect.
(via Boing Boing)