what is a stereogram?
A stereogram is a flat two dimensional image viewed in such a fashion as to produce a three-dimensional effect. Traditionally done with opto-mechanical devices, when such an effect is accomplished with the eyes alone it is an autostereogram.
From birth, we learn to view the world using our normal stereo-vision. With this, each eye focuses on an object, seeing the same object on the same plane, but that small distance between each eye feeds the brain enough different information to give our minds a sense of depth and distance. This phenomena is called stereopsis.
Stereopsis takes place within the brain, and that is key to the stereogram 3D effect. Only, with stereograms, we diverge or cross our eyes, and feed the brain subtle differences in a pair of objects. It is the stereo-pair image/image that is the basis of stereograms. Even the single image random dot stereogram (SIRDS) uses a sophisticated algorithm to create and disguise an offset pair.

also known as: (S.I.R.D.S.)
Single
Image
Random
Dot
Stereograms

PARALLEL VISION: Each eye locks on the image separately but parallel, as opposed to the convergence of everyday normal vision. This is the most commonly used for autostereograms.

CROSS-EYED VISION: Each eye views the image separately, but right eye views left side, and left eye views right.

Learning how to see autostereograms in like learning how to ride a bicycle, or how to swim. Some people learn very quickly, others on the other hand experience difficulties at first, but eventually everyone manages. Once you have got the hang of it, three dimensional viewing becomes increadibly easy too, no matter how long you had to practice at first.

The following tips will help you along:
Sit in a comfortable position in front of your computer screen.
Don't let yourself be distracted, in the beginning, it's best if you are alone. Sit in a

Move your head so close to the screen that the tip of your nose nearly touches it. Stare straight ahead into the distance as though you could see right through the screen. Now, move your head slowly away from the screen. Make sure you don't change the focus of your eyes. Keep looking into the distance. As soon as you see a 3D picture is beginning to form, stay where you are. Don't change your position in front of the screen. If you should happen to lose the three dimensional picture, it is probably because you have subconciously tried to focus on the picture. If that happens, go back to the start of the method and try again.

CLICK HERE FOR EXAMPLES OF AUTOSTEREOGRAMSacted, in the