Every muscle, every sense, every part of you is exercised at once, or in rapid succession, and no part has time to become seriously fatigued; so that you can play hard all the afternoon and never once be uncomfortably tired, though your muscles have done a tremendous lot of work, measured in foot-pounds or "boy-power," in that time.
The good school imitates nature in this respect.

The recitation periods are short, and recesses frequent; a heavy subject is followed by a lighter one; songs, drawing, calisthenics, and marching are mixed in with the lessons, so as to give every part of the mind and body plenty to do, and yet not over-tire any part.
All-Round Training from Work and Play.

Every game that is worth playing, every kind of work that accomplishes anything worth while, trains and develops not merely the muscles and the heart, but the sight, hearing, touch, and sense of balance, and the powers of judgment, memory, and reason, as well.
If you are healthy, you know that you don't need to be told to play, or even how, or what, to play; for you would rather play than eat.

You have as strong and natural an appetite for play as you have for food when you are hungry, or for water when you are thirsty, or for sleep when you are tired.

It is just as right to follow the one instinct as the others, though any one may be carried to extremes.
Some of the most important part of your training and fitting for life is given by plays and games.