[Illustration: FIG. 88.--A TINY BIT OF POTATO, highly magnified, showing cells filled with grains of starch. Cooking bursts these cells.] =Fats= include the fats and oils found in milk, flesh, and plants. A fat, such as tallow, is solid at the ordinary temperature; while an oil, such as olive oil, is liquid at the same temperature. Tallow was oil while it was in the warm body of the ox. Sugar is transformed into fatty tissue as readily as is fatty food itself. =Proteids= are the only foods that contain the tissue-building nitrogen. Protoplasm cannot be formed without nitrogen. We do not often see a pure proteid food, for this food stuff is not so readily separated from foods containing it as are starch, sugar, and fat. Albu_men_, or white-of-egg, is proteid united with four times its weight of water. Pure proteid is also called albu_min_. Coagulation by heat is one test for proteid (Exp. 4). These are the names of proteids, or albumins, found in several common foods: _casein_, the curd or cheesy part of milk; _myosin_ of lean meat; _fibrin_ in blood; _legumin_ in beans and peas; _gluten_, or the sticky part of wet flour; _gelatin_ in bones. Proteid is valuable to the body as fuel as well as a tissue builder. We could burn beans and peas as well as the strictly fuel foods, starch and fat, in an engine, and get heat to move the engine. If one takes up athletics or hard physical labor, he should increase the amount of fats and carbohydrates eaten, but not of proteid. Muscular activity increases the carbon waste but not the nitrogen waste of the body. =Minerals.=--The iron of the blood and the mineral salts in bone (carbonate and phosphate of lime) must enter the body in organic form in order to be used. Water and salt are mineral foods. The body is about two thirds water. The cells must do their work under water. They cannot live when dried. Water enables the blood to flow; and the blood is not only the feeder, but also the washer and cleanser of the tissues. Some persons get out of the habit of drinking plenty of water, and their health suffers thereby. In such a case drinking plenty of water will be safer and more effective than taking poisonous drugs to restore health.