George MacDonald, Restoring Your Spiritual Passion
It is said of one of the famous composers that he had a rebellious son who used to come in late at night after his father and mother had gone to bed. And before going to his own room, he would go to his father's piano and slowly, as well as loudly, play a simple scale, all but the final note. Then leaving the scale uncompleted, he would retire to his room.
Meanwhile the father, hearing the scale minus the final note, would writhe on his bed, his mind unable to relax because the scale was unresolved. Finally, in consternation, he would stumble down the stairs and hit the previously unstruck note. Only then would his mind surrender to sleep once again.