The blood, owing to variations in the external temperature, has a tendency to retreat from the skin through contraction of the capillaries and to engorge the internal organs. Exercise brings the blood to the skin and muscles, causing the waste, broken down by the chemical activity going on every instant of life, to be picked up by the blood and carried to the eliminating organs. Therefore, since the blood is needed in the digestive organs during digestion, active exercise should not be taken immediately after meals. Exercise taken in the proper amount and at proper times uses up the excess of material, benefits digestion, aids the work of the liver and intestines, keeps the circulation active, the waste eliminated, and results in a feeling of vigor and fitness for one’s work whether physical or mental. Exercise should be counted as a necessary part of one’s daily activities—as necessary as eating one’s meals. If faithfully done the habit will be formed and the system will soon call for exercise as it does for food. The young child’s blood circulates freely, his breathing is unrestricted, the waste of the system is fully burned up, potential energy is released, and the result is, he must be active. The effort of the teacher, or of those having the care of children should be, not to restrain the child but rather to direct his activity in advantageous and effective use of his energy. A little child is an object lesson in alternating exercise, sleep, and food. Almost every waking moment a child is squirming, twisting, and turning, using every muscle of his little body, particularly every vital organ. No excess of waste accumulates in his tissues. The adult does not, as a rule, twist or turn or freely stretch the muscles of the vital organs. The child and the animal stretch and yawn to start the circulation whenever they awaken from sleep. This is instinct—Nature’s law. Man jumps out of bed and begins dressing with mind bent on the business of the day. The necessity of oxygen is evident. The body will subsist about forty days on the food stored within it without resupply, but it can endure only a few seconds without oxygen, because heat, occasioned by the chemical action of oxygen, is necessary to keep up the physical activity termed “life.” Carbon dioxid (carbonic acid gas) accumulates and poisons the system. _The necessity of habits of full, correct breathing cannot be too fully emphasized._ The quantity of oxygen daily consumed should equal the sum of all other food elements.[8] Oxygen is necessary in the combustion of fats, starches, and sugars, as it is necessary in the combustion of carbon in wood or coal, and, as explained on pages 123 and 124, oxygen is necessary to keep the body warm.