I argue that interaction is essential to children both for learning a first language and for learning how to use it.  Interaction allows children to discover the forms to use,  and gives them  feedback on those forms for conveying particular meanings,  without disrupting the ongoing exchange.  I will give evidence for the role of interaction in the process of acquisition from (a) parental reformulations, (b) offers of new words, and (c) offers of information about new word meanings.