Stereopsis refers to our ability to perceive depth and see things in 3D. We owe this ability to the fact that our eyes are located beside each other on the front of our skull (as opposed to let's say a horse, whose eyes are located on opposite sides of its skull). Because of this placement, each eye's view of a scene is slightly different from the other. These two different pictures, when sent to the brain for processing, are fused to create a unified three-dimensional picture. Stereopsis allows us to do important everyday tasks that require depth perception and estimation of distance like parking our car, threading a needle, or grabbing a doorknob.