The following two images are autostereograms. They are images that can be seen in 3D if one succeeds in overcoming the eye’s natural coordination of focus and horizontal convergence.
There are two ways that seem to work best in order to achieve the desired effect:
Get very close to the screen, and stare at a specific point on the image. Keep focusing on that point as you slowly move your face backwards away from the screen.
OR
Find a point in the centre of the image, and cross your eyes. Slowly uncross your eyes bit by bit while focusing on the same spot.
In both methods, your vision will “snap” to the right view at a certain point, as your brain registers the depth perception that you are trying to trick it into. At this point the image should appear in vivid 3D, for some people even providing the possibility to look around at specific areas of the image in detail.
(Some people will not be able to see the effect, usually due to one eye being too dominant, or due to another kind of anomaly in their binocular vision).