Once children break into the speech stream, they have two problems to solve. First, they have to map meanings onto words and phrases. For each conceptual domain, they have to find out, first, how to express particular meanings via the words and phrases available in the language spoken around them, and, second, how best to use language to communicate their intentions to others. They must discover how to tailor their utterances for each addressee, taking common ground into account, marking social distinctions appropriately, in order to convey what they mean on each occasion. In solving these two problems, children must look for consistent pairings of situations with utterances or parts of utterances in adult speech. They need to take detailed account of what adults say when and for what purpose. Learning to convey their own intentions is inseparable from learning how to interpret the intentions of others. The prerequisite to this, of course, is breaking into the speech stream to identify recurrent chunks and attach preliminary meanings to them (Chapter 3).

What is the content of children's first utterances – the single words they pick up on and the first meanings they attach to them – in early expressions of their intentions? What is the nature of early vocabularies and their relation to eventual (adult) vocabulary size? And what paths do children follow as they add new words to their repertoires? Do they add words at a steady pace or in spurts? Do they all progress in the same way?