If you have ever seen the inside of a watch or a clock you know how many curious little wheels it has.

And yet a watch or a clock can do but one thing, and that is to tell us the time of day.

The body has a great many more parts than a watch has, and for this reason the body can do many more things than a watch can do.

It is more difficult, too, to learn about the body than about a watch.
~7.~ If we want to know all about a machine and how it works, we must study all its different parts and learn how they are put together, and what each part does.

Then, if we want the machine to work well and to last a long time, we must know how to use it and how to take proper care of it.