Young children learn language at an incredible rate. Whilechildren come prepared with powerful statistical learningmechanisms, the statistics they encounter are also prepared forthem: Children learn from caregivers motivated to communi-cate with them. Do caregivers modify their speech in orderto support children’s comprehension? We asked children andtheir parents to play a simple reference game in which the par-ent’s goal was to guide their child to select a target animal froma set of three. We show that parents calibrate their referringexpressions to their children’s language knowledge, produc-ing more informative references for animals that they thoughttheir children did not know. Further, parents learn about theirchildren’s knowledge over the course of the game, and cali-brate their referring expressions accordingly. These results un-derscore the importance of understanding the communicativecontext in which language learning happens.