Nicolas is diagnosed with autism and he did not have vocal speech. a therapist working with nicolas initially gave him a piece of chocolate any time he made a sound with his lips. this slowly changed until nicolas only received a piece of chocolate for saying complete words, and eventually only for saying complete sentences. in this example, nicolas developed speech skills through the use of

Positive reinforcement
Positive reinforcement is the use of a reinforcing stimulus (chocolate; something desirable) following a desired behavior (speech sounds, words) that makes it likely the behavior will occur again.  The speech pathologist wanted Nicholas to make sounds and would provide positive reinforcement for making sounds.  When she wanted him to say complete words, she would reward him for saying the complete words and eventually sentences.