Here's how it works: Our brains perceive depth based on the slightly different images that each eye sees. By focusing your right eye on the left image and your left eye on the right image (by crossing your eyes) your brain tells you that the slight difference between the two means that there is a change in depth. The perfect optical illusion!
When you cross your eyes, to begin with There will be 4 pics, 2 moving away from each other and one moving towards each other. Cross your eyes just enough until there are only 3 pictures and the one in the middle will seem to have depth.
I can only do it when I hold something thin in front of my eyes (like a paperclip) and sort of "train" my eyes to cross properly. At first, I could cross the two images, but they were out of focus. When I got the image to focus, the effect blew my fuckin' mind.