This activity shows how we rely on both our eyes in order to determine depth and distance. This property is termed as binocular vision. As shown in the video it is easy to put a cap on a pen when we use both our eyes but difficult to do so with only one eye. This is because each eye produces a slightly different image and our brain process these two images in order to estimate information about depth and distance. With only one eye open the brain cannot figure out depth and distance easily.