When children learn a language, they learn the grammar as well as words or vocabulary. No one teaches them; children just " _ ". Before babies begin to produce words, they produce sounds. Some of these sounds will remain if they occur in the language being learned, and others will disappear. This is called the "babbling stage". A child does not learn the language "all at once". The child first speaks only one-word "sentences". After a few months, the two-word stage arises. During this stage, the child puts two words together. These two-word sentences have definite patterns and express grammatical and meaningful relationships. Still later, in the telegraphic stage, the child will produce longer sentences. These longer sentences are mainly made up of content words. The child's early grammar lacks many of the rules of the adult grammar, but gradually it will become perfect. All normal children everywhere learn language. This ability is not dependent on race, social class, geography, or even intelligence  . This ability is uniquely   for human. This passage is mainly about   _  .
differences between a child's language and an adult's ways of teaching babies to talk children's learning of the mother language the importance of learning foreign languages