You all know enough to eat when you are hungry and to drink when you are thirsty, even though you don't always know when to stop, or just what to eat.

You like sunny days better than cloudy ones, and would much rather breathe fresh air than foul.

You like to go wading and swimming when you are hot and dusty, and you don't need to be told to go to sleep when you are tired.

You would much rather have sugar than vinegar, sweet milk than sour milk; and you dislike to eat or drink anything that looks dirty or foul, or smells bad.
These inborn likes and dislikes--which we call _instincts_--are the forces which have built up this wonderful body-machine of ours in the past and, if properly understood and trained, can be largely trusted to run it in the future.

How to follow these instincts intelligently, where to check them, where to encourage them, how to keep the proper balance between them, how to live long and be useful and happy--this is what the interesting study of physiology and hygiene will teach you..