Years ago, in the pioneer days of aviation, a pilot was making a flight around the world. After he had been gone for some two hours from his last landing field, he heard a noise in his plane, which he recognized as the gnawing of a rat. He realized that while his plane had been on the ground a rat had gotten in. For all he knew the rat could be gnawing through a vital cable or control of the plane. It was a very serious situation. He was both concerned and anxious. At first he did not know what to do. It was two hours back to the landing field from which he had taken off and more than two hours to the next field ahead. Then he remembered that the rat is a rodent. It is not made for the heights; it is made to live on the ground and under the ground. Therefore the pilot began to climb. He went up a thousand feet, then another thousand and another until he was more than twenty thousand feet up. The gnawing ceased. The rat was dead. He could not survive in the atmosphere of those heights. More than two hours later the pilot brought the plane safely to the next landing field and found the dead rat.