Young children initially appear to regard an entire word as if it were a single sound. However, as their vocabulary expands between 15 and 21 months of age, keeping track of a large store of independent sounds becomes very difficult for them to manage. To learn more words, children must begin to break words into smaller number of simpler units, which are sounds that can be used in different combinations to make up many other words. This feature of language can be referred to as _____________.