Learning to speak languages seems to be a very mysterious process. For a long time, people thought that we learned a language only by imitation and association. For example, a baby touches a hot pot and starts to cry. The mother says, "Hot, hot!" And the baby, when it stops crying, imitates the mother and says, "Hot, hot!" However, a famous expert in language pointed out that although children do learn some words by imitation and association, they also combine words to make meaningful sentences in unique and unlearned ways. He suggested that human infants are born with the ability to learn language. This theory explains the potential that human infants have for learning language. But it does not really explain how children come to use language in particular ways.