We perceive depth because our brains are constantly receiving two slightly different images from the right and left eye and fusing them to see 3D images.
In order to reproduce this depth perception with 2D pictures, two different images must be provided, one to the left eye and one to the right. The brain is then fooled into “seeing” depth in the otherwise flat pictures.
There are a number of ways to view 3D images, but they all rely on one image going to the left eye and a slightly different image to the right. This separation may be done through polarized filters, colored filters, or physical isolation of what each eye can see.
(If you do not own a pair of red/cyan glasses, send a note with a SASE to 3D Glasses, PO Box 227, Forest Knolls, CA 94933 and we’ll be happy to send you a pair.)