There is a physical basis for the remarkable alterations in the minds and morals of this age. The infant grows very rapidly at first, but with a diminishing rate until about the twelfth year. Then, almost suddenly, the rate of growth increases again, and in four or five years most children have attained nearly their full physical growth. Associated with this great physical growth is the fact that some organs grow much faster than others, so that the proportions of an adult come to be very different from those of a child. In the meanwhile, however, there has been a great strain on the system, because, apart from the demands of the general body growth, some of the organs have not been able to keep up with the special demands made upon them. For example, the growth in body weight and in muscle may proceed more rapidly than the proportionate growth of the lungs or the liver, or the weight may increase more rapidly than the proportionate strength of the muscles. Moreover, the nervous system is developing at a more rapid rate, probably, than the other systems of organs, and this strain shows itself in various ways that are disagreeable to adults with fixed habits and standards. All of these changes are intimately bound up with the development of the sex organs and with the approach of sexual maturity. A graceful child becomes awkward and a well-mannered child comes to act rudely and to speak quite unlike his former self. These changes are related to the fact that with the development of the nervous system there arise impulses for hundreds of new kinds of movements which the child can learn to suppress or to control only with the passing of time. This is the age at which the child is exposed to the acquirement of many undesirable muscular habits, such as various kinds of fidgetings, biting of the finger-nails, twirling of buttons, wrinkling of the forehead, shruggings, swaying the body, rolling the tongue, tapping with the fingers or the feet, and so on. Nearly a thousand of these uncontrolled or "automatic" movements have been described in children of this age. Of course, any of these movements that produce sounds or that catch our eye are very annoying to us, and if we have never nagged before, we are likely to begin now by saying _Don't this_ and _Don't that_, for we have never been tempted like this before. But nagging is not what is called for.