True concentration of mind means the ability to drop every subject but that centred upon. Tell one man to concentrate his mind on a difficult problem until he has worked it out,--he will clinch his fists, tighten his throat, hold his teeth hard together, and contract nobody knows how many more muscles in his body, burning and wasting fuel in a hundred or more places where it should be saved. This is _not_ concentration. Concentration means the focussing of a force; and when the mathematical faculty of the brain alone should be at work, the force is not focussed if it is at the same time flying over all other parts of the body in useless strain of innumerable muscles. Tell another man, one who works naturally, to solve the same problem,--he will instinctively and at once "erase all previous impressions" in muscle and nerve, and with a quiet, earnest expression, not a face knotted with useless strain, will concentrate upon his work. The result, so far as the problem itself is concerned, may be the same in both cases; but the result upon the physique of the men who have undertaken the work will be vastly different. It will be insisted upon by many, and, strange as it may seem, by many who have a large share of good sense, that they can work better with this extra tension. "For," the explanation is, "it is natural to me." That may be, but it is not natural to Nature; and however difficult it may be at first to drop our own way and adopt Nature's, the proportionate gain is very great in the end. Normal exercise often stimulates the brain, and by promoting more vigorous circulation, and so greater physical activity all over the body, helps the brain to work more easily. Therefore some men can think better while walking. This is quite unlike the superfluous strain of nervous motion, which, however it may seem to help at the time, eventually and steadily lessens mental power instead of increasing it. The distinction between motion which wholesomely increases the brain activity and that which is simply unnecessary tension, is not difficult to discern when our eyes are well opened to superfluous effort. This misdirected force seems to be the secret of much of the overwork in schools, and the consequent physical break-down of school children, especially girls. It is not that they have too much to do, it is that they do not know how to study naturally, and with the real concentration which learns the lesson most quickly, most surely, and with the least amount of effort. They study a lesson with all the muscles of the body when only the brain is needed, with a running accompaniment of worry for fear it will not be learned.