When we have learned to choose and cook wholesome and appetizing food we have not solved the whole problem of successful feeding. It is possible to make people sick with good food, if it is badly selected and fed at wrong times or in unsuitable amounts. Whether children grow to their full size and strength depends more upon the choice of their food than upon any other one thing. The effect of food is strikingly shown in the case of the white rats in Fig. 74. The two upper ones are the same age. Both had the same mother, lived in the same kind of clean cages, and had plenty of food, but the diet of the upper was good for growth, while that for the middle one was not. It remained perfectly well, but became stunted because of the character of its food. You can see that it resembles the lowest one in the illustration, which is only one fourth as old. In this chapter we shall consider how and when and in what amounts to serve food so that every one may get from it the fullest benefit in both health and happiness. [Illustration: FIG. 74.——The effect of food on growth. Reprinted from publication of the Carnegie Institution. _Courtesy Professor Lafayette B. Mendel._] In Chapter I we learned that the body is a working machine whose first requirement is fuel. Hence the first consideration in the diet is to have the proper amount of fuel for each day, to provide energy for the constant internal work that keeps the body alive, and for the variable external work which may be so light as to consist of the few movements that one makes lying in bed, or sitting quietly; or so hard as to exercise many muscles, as playing tennis, bicycling, or swimming. [Illustration: FIG. 75.——Respiration calorimeter, open. From the “Journal of Biological Chemistry.” _Courtesy of Professor Graham Lusk._]