Teachers of second languages tend to forget that even first-language learners commit a multitude of errors as they progress over a period of years from initial utterances to adult speech patterns. The external, mechanistic view is that such errors result from an imperfect imitation of the adult sample. Correct speech is shaped over a period of years by reinforcing desired responses. The internal view is that during the acquisition process the learner's self-generated grammar system does not conform to that of the adult language system. Given the time, practice, and cognitive maturity, learners will gradually bring their language into line with that of other adult speakers.