Human development is categorized into nine stages: prenatal period, infancy, babyhood, childhood, puberty, adolescence, adulthood, middle age, and old age. Childhood is further divided into early childhood and late childhood. Adolescence is divided into early adolescence and late adolescence. Each stage involves specific physical, mental, social, and emotional changes and characteristics. For example, the prenatal period involves rapid physical development, infancy involves total dependence on parents, and puberty involves sexual organ development and a bridge to adolescence.