This means that the teacher must know the whole subject so thoroughly
that the right questions come to him easily and naturally, and in the
right order to bring out the successive steps of the lesson in their
logical relations.

The difference between a related series of questions and an unrelated
is shown in two lists which follow. Both deal with the same
subject-matter, a physiology lesson on respiration. The questions of
the first list are not themselves faulty, but there is no continuity
among them; one does not grow out of another so as to "develop" the
subject in the minds of the class.

What change takes place in the air while in the lungs?

What change takes place in the blood while in the lungs?

How many cubic inches of air will the lungs contain?

How much of this cannot be expelled by breathing out?

How many times do we naturally breathe in a minute?

What are some of the effects of breathing impure air?

How is the oxygen carried by the blood?

What is animal heat?

What is the temperature of the body?