Anya Kamenetz previews Deb Roy's amazing mission to capture every word uttered in his child's first five years, as a way of understanding the birth of language:

Most moving of all was the precise mapping of tight feedback loops between the child and his caregiversfather, mother, nanny. For example, Roy was able to track the length of every sentence spoken to the child in which a particular word--like “water”--was included. Right around the time the child started to say the word, what Roy calls the “word birth,” something remarkable happened.

“Caregiver speech dipped to a minimum and slowly ascended back out in complexity.” In other words, when mom and dad and nanny hear a child speaking a word, they unconsciously stress it by repeating it back to him all by itself or in very short sentences. Then as he gets the word, the sentences lengthen again. The infant shapes the caregivers’ behavior, the better to learn.