In the world infants encounter, there are various social interactions. Some people might approach, while others might talk nearby or stare at their smartphones. Infants also eat, sleep, and play with others every day. Infants acquire language based on learning in such an environment.

Use of language is supported by various cognitive processes. Paying attention to something with others and understanding others’ intentions is the foundation of communication. Learning a language format, generalizing meanings, and memorizing patterns allow the use of language as symbols in the same manner as others.

To utter linguistic signals, it is essential to control fine physical movements. Articulation in the first language might appear easy and obvious, but that is only because we have mastered the necessary movements. In the case of spoken language, movements of the glottis, tongue, and lips, in the case of sign language, movements of the hand, fingers, and upper body are repeated many times to achieve fine control.