He is made tired with five minutes' labor.

He is satisfied with five minutes' rest.

He will ride his rocking-horse, if alone, a short time, and then he comes to you to ask you to tell him a story.

While listening to the story, his muscles are resting, and the force is spending its strength in working the mechanism of the brain.

If you make your story too long, the brain, in turn, becomes fatigued, and he feels instinctively impelled to divert the vital force again into muscular action.
If, instead of being alone with his rocking-horse, he has company there, he will _seem_ to continue his bodily effort a long time; but he does not really do so, for he stops continually, to talk with his companion, thus allowing his muscles to rest for a brief period, during which the vital force expends its strength in carrying on trains of thought and emotion through the brain.
He is not to be blamed for this seeming capriciousness.