When children reach about 15–18 months of age, language acquisition flourishes. There is a surge in word production resulting from the growth of the cortex. Infants begin to learn the words that form a sentence and within the sentence, the word endings can be interpreted. Elissa Newport and colleagues (1999) found that humans learn first about the sounds of a language, and then move on to how to speak the language. This shows how infants learn the end of a word and know that a new word is being spoken. From this step, infants are then able to determine the structure of a language and word.