It is a familiar principle that attention to the thought of a movement tends to start that very movement.

I defy any of my readers to think hard and long of winking the left eye and not have an almost irresistible impulse to wink that eye.

There is no better way to make it difficult for a child to sit still than to tell him to sit still; for your words fill up his attention, as I had occasion to say above, with the thought of movements, and these thoughts bring on the movements, despite the best intentions of the child in the way of obedience.

Watch an audience of little children--and children of an older growth will also do--when an excited speaker harangues them with many gestures, and see the comical reproduction of the gestures by the children's hands.

They picture the movements, the attention is fixed on them, and appropriate actions follow.
It is only the generalizing of these phenomena that we find realized in the boy or girl of the motor type.