The calm of late childhood gives way to the upheaval of adolescence just as the earlier security of infancy gave way to the emergence of childhood. This upheaval arises when the equilibrium of late childhood is disrupted by changes in body, mind, and social demands. Adolescence begins when the child's body and mind spurt ahead to new levels. Adolescence begins with a spurt of physical growth that is more or less rapid in different children. Beginning at approximately the age of twelve, the child gains height and weight and, within a few years, has reached his or her adult size. The physical changes of adolescence refocus concern on the body as the center of identity. The major contribution of the cognitive changes of adolescence comes from Piaget; especially from his concluding chapter in The Growth of Logical Thinking from Childhood to Adolescence. As Piaget repeatedly stresses, adolescence is a time when the possibility of assuming adult roles is central.