Communication is crucial for a social species and particularly for human infants who are dependent on others for years. When children start to learn language, they first learn to ＿＿（41）＿＿ words (as well as emotional tones, etc.) and only later produce words or the signs of sign language. There is no good way to ＿＿（42）＿＿ exactly what an infant understands, so almost all of the research has been about what words children produce. As an important first step in producing language, a child must learn to produce the sounds of her language, ＿＿（43）＿＿ she already can perceive. These efforts are known as babbling, and similar practice takes place in the hand movements of children learning signed languages. ＿＿（44）＿＿ the child can produce enough of the basic phonemes of her language, the production of words starts and there is no looking back. The generally accepted ＿＿（45）＿＿ is that a child learns an average of ten new words a day between the ages of 2 and 12. At the beginning, the learning is slower, more like ten words per week. With these early words, the child is able to name her experiences and communicate with her caregivers.