Aspect of the function of human eyes (specifically the retina) and how the brain holds an image for a small fraction of a second enabling a series of rapidly changing still images to generate the appearance of motion; principle of human sight that makes video and movies possible. When an image is flashed up at our eyes, our brain takes it and holds onto it for a short time. If another image follows close behind it, our brains meld the two images (the first image it has been holding in storage along with the new image). If this happens in quick succession many times each second, the brain blends the images into a continuous stream. The image stream appears to us as motion. This is the principle behind movies and television. By flashing up enough images in quick succession, the images blend together to form motion.