This is without doubt one of the coolest illusion.
1. Relax and concentrate on the 4 small dots in the middle of the picture for about 30-40 seconds
2. Then, take a look at a wall near you (any smooth, single coloured surface)
3. You will see a circle of light developing
4. Start blinking your eyes a couple of times and you will see a figure emerging...
5. What do you see? Moreovere, who did you see?