In the first months of life, infants coo and babble; by their 24th month, toddlers can speak some grammatically correct sentences. In the first months, infants respond to the human voice; by their 24th month, toddlers comprehend the meaning of some prepositions. Language acquisition lies at the nexus of impressive accomplishments in the percep-
tual, cognitive, and social spheres of development. Human language is also a multivariate matter of study, because it simultaneously involves sounds, semantics, and syntax, each of which has both productive and receptive constituents.