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Very early childhood, generally referring to the period up to age two. During this important formative period, children begin to develop habits and behavior patterns, and acquire many basic skills, including speech.

Compared to the young of other mammals, human infants are precocious in some ways—notably sensory development—and relatively helpless in others, such as physical strength and mobility. At birth, the average American infant weighs approximately 7.5 pounds (3.37 kg), although a baby born 28 weeks after conception may weigh as little as two pounds (0.9 kg). The average length of an American newborn is about 21 inches (53 cm).

Infants are born with several reflexes that are activated by particular stimuli, such as the grasping reflex when a finger...