How many wolves do you see?

Circles or spiral...

Does the square move..

How many faces do you see?

This mind trick fools you into counting one to many prongs on the fork.

Cover your right eye and do the exercize. Then switch eyes and watch what happens.

Ok this one is just cool..

That was cool...

The following demonstration, by Daniel White, is a great use of the colour aftereffect illusion. The image below will give an example of a colour you will never see on your computer screen. Stare at the dot in the centre of the red circle for three to five minutes. You will see the edges of the circle begin to glow, but do not stop staring at that point - keep on. After a few minutes move your head back and you will see a glowing ring around the red circle. The colour you are seeing is a cyan which will be perceptually totally different from any of the colours you have seen on your computer. The colour gradient to the right shows the shades of 'cyan' which your computer can produce, none of which match the colour you see with this effect.