There are certain requirements of proper procedure, however, which should never be neglected by any one who experiments with young children.
In the first place, the child is peculiarly susceptible to the appeals of change, novelty, chance, or happy suggestion; and often the failure to respond to a stimulus is due to distraction or to discomfort rather than to lack of intrinsic interest.

Again, fatigue is a matter of considerable importance.

In respect to fatigue, I should say that the first signs of restlessness, or arbitrary loss of interest, in a series of stimulations, is sufficient warning, and all attempts at further experimenting should cease.

Often the child is in a state of indisposition, of trifling nervous irritability, etc.; this should be detected beforehand, and then nothing should be undertaken.

No series longer than three trials should be attempted without changing the child's position, resting its attention with a song, or a game, etc., and thus leading it fresh to its task again.