How do we acquire language?

This was the burning question for Deb Roy, MIT cognitive scientist. But instead of merely writing a paper or a book on it, he has recorded on video the first 5 years of his son's life; focussing it on his progress in learning how to speak. This online article describes Deb's real-time action research and includes a video showing how the boy learnt to say  the word "ball" over a period of months. And here is the video of his recent TED talk.