The scaly cells on the surface of our skin, our hair, and the tips of our nails are the only parts of us that live in air. In fact, over five-sixths of the weight and bulk of our bodies is made up of water. Some one has quaintly, but truthfully, described the human body as composed of a few pounds of charcoal, a bushel of air, half a peck of lime, and a couple of handfuls of salt dissolved in four buckets of water. The reason why nearly all our foods, as we have seen, contain such large amounts of water is that they, also, are the results of life--the tissues and products of plants or animals. Water Frees the Body from Waste Substances. Water in the body, then, is necessary to life itself. But another most important use is to wash out all the waste substances from the different organs and tissues and carry them to the liver, the kidneys, the lungs, and the skin, where they can be burned up and got rid of. We must keep our bodies well flushed with water, just as we should keep a free current of water flowing through our drain-pipes and sewers. It Keeps the Body from Getting Over-heated. In summer time, or in hot climates the year round, an abundant supply of water is of great importance in keeping the body from becoming overheated, by pouring itself out on the skin in the form of perspiration, and cooling us by evaporation, as we shall see in the chapter on the skin. The Meaning of Thirst. None of us who has ever been a mile or more away from a well, or brook, on a hot summer's day needs to be told how necessary water is, for comfort as well as for health. The appetite which we have developed for it--_thirst_, as we call it--is the most tremendous and powerful craving that we can feel, and the results of water starvation are as serious and as quick in coming as is the keenness of our thirst. Men in fairly good condition, if they are at rest, and not exposed to hardship, and have plenty of water to drink, can survive without food for from two to four weeks; but if deprived of water, they will perish in agony in from two to three days. [Illustration: THE CHAINED CUP An "Exchange" for disease germs.] We should Drink Three Pints of Water a Day. Although all our foods, either as we find them in the state of nature, or as they come on the table cooked and prepared for eating, contain large quantities of water, this is not enough for the needs of the body; to keep in good health we must also drink in some form about three pints, or six glassfuls, of water in the course of the day. Part of this goes, as you will remember (p. 16), to dissolve the food so that it can be readily absorbed by our body cells in the process of digestion. WHERE OUR DRINKING WATER COMES FROM