Halliday (2008: 157):
Once both grammars are in place, however, by the time the child enters secondary school, the two form complementary constructions of reality. Technical knowledge is construed almost entirely in these metaphoric terms. But since every adolescent started life as a child, the primary commonsense grammar is still in place, and the picture of the world that is being built up by language is not just the reconstructed version but a composite. The result is a multi-faceted perspective on reality which, if you come to terms with it, gives a plausible representation of the contemporary human environment.