“When babies babble, they are testing out their vocal apparatus to see what sounds they can make. They're also testing out their brains' language faculties—seeing whether people around them respond more to one sound or another. Imagine learning an instrument before you even have an idea what music is. You play a note or two, hear it, see if you like it, see if your audience likes it, and then you play more. Except that the instrument is located in your own chest, throat, and head. Meanwhile, your brain is rewiring itself with simple sorts of rules for communication by paying careful attention while your main caregiver talks to you.”