THE TECHNIQUE OF LESSONS 201 Prevention of Errors in Using Material. The most efficient prevention of misuse of the material lies in its presentation at the right moment of the child's development. Another guarantee of proper use lies in the exactness of the presentation. If, notwith- standing this, a mistress sees material being used in such a way as to render it valueless for its purpose, that is, that it does nothing towards the development of the child's intelligence, she must put a stop to the work. She will do this with the utmost sweetness if the child submits quietly and with good temper, but if he shows a tendency to behave badly she will restrain him energetically, not so much as a punishment for noise and disorder, but offering him the help of her authority. Authority, in fact, becomes in such a case the support which is needed by the child, who having lost control of himself owing to temporary lack of balance, needs a strong support to which he can cling, just as. one who has stumoled needs to hang on to some- thing to maintain his balance. The work of helping means, at such a moment, the friendly hand of the strong held out towards the weak. When the child is working he is like a person poised in perfect equilibrium, and it is material which he needs with which to employ himself, just as the body striving after perfect mobility would require a gymnasium. We must distinguish clearly between two kinds of mistakes which the child may make. First, we have the error which is controlled by the material itself, and which arises from the fact that the child, quite willing to carry out exactly an exercise which he knows well, yet does not succeed because of his immature power of execution or because his senses do not appreciate the various stimuli, or because he cannot execute definite movements for which his mechanism is not yet far enough developed. For example, he makes a mistake in putting the cylinders into the holes, because he does not yet distinguish the differences between them; or for a similar reason, he places a large cube over a small one in building the tower, and so on.