Put your 3D glassses on and look at an anaglyph card. The left eye looking through the red filter sees a blue green image in the anaglyph. The right eye looking through the blue filter sees a red image in the anaglyph. This "trick" on your eyes sends two different images to your brain. Your brain assembles the two images into a new picture that appears to have perpective, it looks like it is 3-D,