Delayed fatigue, then, is an important thing to notice in ourselves and to bring home to the people that we are trying to help. I suppose one could say that _a great part of our business in social work is to call people's attention to things_; if they have recognized them before, they will perhaps get a lesson out of what we say. Such matters are _referred fatigue_, _delayed fatigue_, _accumulated fatigue_,--familiar enough, only the person does not act on them because he does not notice them. The fatigue-rest rhythm, the alternation of fatigue and rest, I have already phrased by the metaphor of earning and spending. You can phrase it also by a metaphor very close to the physical facts as we know them, the metaphor of building up and tearing down. During the daytime, from the point of view of physiology and the workings of the body, we burn up tissue. In us oxidation processes are going on which are really burning, as really as if we saw the flame. Tissue is being destroyed, broken down, going off in the form of heat, energy, and life. That is good in case it is followed, as it should be, by a period of rest in which we build up. Presumably, if we could see with adequate powers of the microscope or powers of observation of some sort, what goes on during rest, we should see a perfect fever of rebuilding all that we have torn down during the day. People often say, "Shall I take exercise?" Yes, but remember that half of the process of taking exercise is getting rested afterwards. It will do you good provided you rest after it, provided what has been torn down in exercise is replaced by sufficient tissue or fresh power in rest. The English studies of fatigue to which I have referred have been of great importance because, so far as I know, they are the first attempt we have had in the way of testing when men or women in industry are too tired and how much too tired they are. I do not suppose that any employer of labor would want for his own profit or for more than a short time to overwork people in this sense, if he had the facts called to his attention. If he realized what he was doing, he would not want to break down his working force any more than he would to spoil his machinery. But some employers are careful of their steel machinery and careless of their human machinery. They will continue to be so, I fear, until we know more about fatigue.