Research shows that infants can discern very subtle nuances between phonemes (functional units of speech sound). Babies detected the differences between "pa" and "ba" or "da" and "ga." Babies have quite well-tuned perception of speech sounds, and they appear to organize sounds in a way similar way as adult speakers will, a phenomenon known as "categorical perception." These sounds the babies experiment with in classifying and organizing are said to be the same as those that form the basis for many speech systems around the world. Noam Chomsky's work on universal grammar clearly indicates that humans are born with the capacity for language development and do not, as previously thought, acquire it by exposure.
Sounds that are heard from babies are mainly crying. Researchers noticed that by 4 months of age, a baby's vocabulary has expanded considerably. Babies are now smiling at caregivers, and they are making the alluring cooing noise to parents. Sounds are not totally speech-like at this time, but there are many open mouthed vowel-like sounds and the occasional consonant sound as well but without the finishing properties that make for a full syllable. At some time between 4 and 10 months, baby begins making more speech-like syllables with vowel and consonant sounds, in what is called "canonical babbling." From 6 to 8 months the baby will practice sounds she can make, and although not all of these are actual words, research has shown that some cultures sound alike at this stage. Babbling consists of "bababa" or "dadada" or "mamama." It hardly seems a coincidence therefore, that humans have adopted those sounds as names for Mother and Father.
Between 9 and 15 months, depending on the child's intelligence, the baby makes discernible sounds that are starting to sound like adult speech. The meaning conveyed by the baby is questionable, but when the baby sees that it brings attention, the sound is repeated until the caregiver responds. Babies use words for attention at first, and new words will come and go, as they may be situation specific, such as "dog." After six or so months of slow growth, a phenomenon called the "word spurt" happens as the child develops an interest in what things are called, to an estimated nine words a day acquired from age 2 to 18.
The meaning of a word matters little to children, and they may "overgeneralize" to refer to things that are beyond their knowledge or understanding, such as calling all men "Daddy" or all animals "doggie." Objects are put into a class, such as all bright lights may be called "moon" for a child who learned the word. Children will often name objects at the "basic object" level, recognized as an intermediate level of abstraction. They refer to "dog" instead of the specific type of dog, such as a collie. Most parents will use these naming practices for their children around the ages of 2 years.
The ability to use syntax and generate grammar and sentences separates humans from other creatures. At 18 to 24 months, most toddlers can produce a two-word sentence. They string familiar simple words together until their first sentence emerges under a single fluent intonational group. First sentences are not deep but show a major step forward in expression. Nouns and verbs are generally produced first, and functional words such as articles, prepositions and auxiliary verbs are rarely seen. Some tests have shown, however, that children are sensitive to the words that they omit.