Perhaps the simplest of the child’s models of language, and one of the first to be evolved, is what we may call the INSTRUMENTAL model. The child becomes aware that language is used as a means of getting things done [...] Language is brought in to serve the function of ʻI wantʼ, the satisfaction of material needs. Success in this use of language does not in any way depend on the production of well-formed adult sentences; a carefully contextualized yell may have substantially the same effect, and although this may not be language there is no very clear dividing line between, say, a noise made on a commanding tone and a full-dress imperative clause.
- Halliday (1973), Pag. 11-12