[WhiteXFire]

The way the 3D illusions work is based on the way our eyesight works. When we look around, we don't actually see in 3D, but instead two separate 2D images...one with each eye. Our brain then interpretes how far objects in our vision are from us by the relative placement of them in each image. In other words, each eye sees the same picture, but from a slightly different angle. Something that is closer to us will be shifted more between the two images than something further away. This is why humans and predators such as wolves/lions have such good depth perception...stereoscopic vision. This differs from other animals who have their eyes on the sides of their head, which gives them much better peripheral vision and perception of motion in their visual field.

Here are some more images based on the same illusion, but actually separated out rather than in one image. The basic idea here is the same, to cross your eyes so that the two images come together in the middle so your brain can calculate the relative depth of different parts of the image.