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Viewing stereo images
We see objects in 3D because each eye sees an object from a slightly different angle, and the brain puts together the two images to determine depth or distance. Of course, there are other clues that give an illusion of depth, such as occlusion, where one object covers part of another object.
Stereoscopy fools the brain into thinking there is depth by feeding a slightly different image to each eye. Most stereo formats require special lenses or other display tools to isolate what each eye sees.
In the cross-eyed format, the left image is intended for the right eye, and the right image is intended for the left eye. Neil Creek has a wonderful tutorial about how to view cross-eye stereos.