Shaping is a reinforcement technique that is used to teach animals or people behaviors that they have never performed before. In this method, the teacher begins by reinforcing a response the learner can perform easily, and then gradually requires more and more difficult responses. For example, to teach a rat to press a lever that is over its head, the trainer can first reward any upward head movement, then an upward movement of at least one inch, then two inches, and so on, until the rat reaches the lever. Psychologists have used shaping to teach children with severe mental retardation to speak by first rewarding any sounds they make, and then gradually requiring sounds that more and more closely resemble the words of the teacher. Animal trainers at circuses and theme parks use shaping to teach elephants to stand on one leg, tigers to balance on a ball, dogs to do backward flips, and killer whales and dolphins to jump through hoops.