Imagine a ball in real life and a ball on TV. Infants are developing 3-dimensional vision. The world of the screen exists in 2 dimensions, so the ball is just a flat, shaded circle. If you roll a ball across the floor it proceeds in a single motion, slowing gradually until it stops. The same action on TV is broken up—you see the ball leave someone’s hand, then there’s a shot of it in motion, then a picture of the ball at rest. If your infant wants to grab a ball in real life he’ll lunge for it, grasp at it, or crawl after it. The stuff on the screen just disappears, to be replaced by other stuff; you can never get your hands (or mouth) on it.