Try staring at this image and see if part of it becomes 3D. If you stare long enough, the circle in the centre should project itself forwards, away from the background. The black and white checks in the circle may also move, giving the illusion that the circle is a ball, floating.
This type of optical illusion is effective because of the way our brains perceive objects and particularly black and white patterns within objects. The circle acts differently, and is independent, from its background because of the change in its structure and the way in which we understand it.
Archive of illusions.org: