Learning a second language involves learning a new culture and way of thinking. Children begin learning language from birth through making noises and cries. By ages 3-4, they have mastered the essential communication skills of their first language. The process of acquiring a first language is natural and not linked to intelligence, and occurs in stages from prelinguistic sounds to two-word phrases to grammar development aided by adults. Second language acquisition theories include behaviorist views of imitation, cognitive views of rule deduction, and the critical period hypothesis that language learning is best before puberty due to brain development.