Language development requires normal development of organs, auditory and language centers in the brain. Language is the mental process of expressing thoughts and ideas, and words, sounds, and visual signals, postures and gestures are all part of language. Language development goes through three stages: articulation, comprehension and expression. The first cry of a newborn is the earliest articulation. At 2 months, the baby starts to produce some a, o, u, e and other similar back vowels, followed by the consonants n, k, p, m and so on. Around 5 months, the infant enters the yapping stage and makes some repetitive syllables similar to those used in adult language. For example, ba-ba-ba, ma-ma-ma, 10 days after birth, newborns can distinguish between speech sounds and other sounds. At 8 or 9 months of age, infants reach a peak in the frequency of yada yada speech and begin to imitate adult speech by making four sounds. They show that they can understand some adult words and respond accordingly, i.e. they form the first word-action conditioned reflexes. For example, if mom says clap your hands, he will clap his hands, etc. Infants around 11 months of age are a rapid period of word-action conditioned reflex formation, and begin to be able to extricate themselves from compound situations, i.e., they can use words as signals and elicit corresponding responses to understand the meaning of a particular word. At this time, yawning declines. Children between the ages of 1 and 1.5 years old have a rapidly developing ability to understand adult language. For example, if an adult asks, “Where’s the dog?” Infants and toddlers around 1 year old can say meaningful words or short sentences of three or four words to express their wishes, thoughts, and interactions with others. Around 2 years old, they can say two- or three-word phrases. The semantic meaning is clearer than the word sentence, but the structure is incomplete; 3 years old is the critical period for expressing language, and the vocabulary can reach about 1,000, but at this time the pronunciation is not accurate enough and the syntax is not proper enough. At the age of 5, children can master 2,200 to 3,000 words.