So we must always be careful to have plenty of pure air to breathe.
~4.

We should Exercise the Brain.~--What do we do when we want to strengthen our muscles?
We make them work hard every day, do we not?
The exercise makes them grow large and strong.

It is just the same with our brains.

If we study hard and learn our lessons well, then our brains grow strong, and study becomes easy.

But if we only half study, and do not learn our lessons perfectly, then the study does not do our brains very much good.
~5.