Infants can differentiate most sounds soon after birth, and by age 1, they become language-specific listeners. But researchers are still trying to understand how babies recognize which acoustic dimensions of their language are contrastive, a linguistics term that describes differences between speech sounds that can change the meanings of words. For example, in English, the letters b and d are contrastive, because changing the b in “ball” to a d makes it into a different word, “doll.”