By 4 months, many babies begin listening for the individual sounds in words, noticing how vowels and consonants combine into syllables, words, and sentences. Their babbling might sound like gibberish, but these early attempts at language begin to incorporate the rhythms and characteristics of their native languages. Babies’ responses to sounds also serve as experiments with different sounds, tones, and combinations of sounds. They tend to follow the lead of their caregivers, sometimes repeating a sound for a day or more before experimenting with a new sound.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.