Hunger is a peculiar craving which we are accustomed to say comes from the stomach. It is the business of the stomach to change such food as we take into it in such a way that the rest of the organs of the body which we have for the purpose can make blood out of it. When you feel the sensation of hunger, it means that the blood-producing system is calling on the stomach to furnish more blood-making material. The stomach prepares the food for blood production by mixing with it certain juices which the stomach is able to supply. As soon as the stomach is then called upon to supply more blood-making material, it goes to work on what is in the stomach and begins mixing things. If, however, there is nothing in the stomach, the craving which we call hunger is produced. It is, therefore, then not altogether the stomach which makes us hungry, but the parts of our body which actually turn the food into blood after the stomach has prepared it. To prove this it is only necessary to say that the sensation of hunger will stop if food which is easily absorbed and, therefore, does not need the preparation which the stomach generally gives, is introduced into the system through other parts of the body, as, for instance, by injecting it into the large intestine, which is a part of the body, the food passes through after it leaves the stomach ordinarily. What Makes Us Thirsty? Thirst is a sensation of dryness and heat which is generally communicated to us through the tongue and throat. The sensation of thirst can be artificially produced by passing a current of air over the membranes which cover the tongue and throat, but thirst is naturally due to a shortage of water in the body. The human body requires a great deal of water to keep it in condition, and when the supply becomes low a warning is given to us by making the membranes of the tongue and throat dry. In connection with thirst, however, as in the case of hunger, where the warning is given by the stomach, thirst will be appeased by the introduction of water, either into the blood, the stomach or the large intestine, without having touched either the tongue or throat, which proves that it is not our tongue or throat that is thirsty, but the body itself. What Is Pain and Why Does It Hurt? Pain is the result of an injury to some part of our bodies, or a disturbed condition--a change from the normal condition. Pain is caused by nerves in the body. The network of nerves coming in big nerves from the back bone or spinal chord branches out in all directions, and near the surface of the skin they spread out like the tiny twigs of a tree, covering every point of the body. Some parts of our bodies are more sensitive than others. That is because the nerves are then nearer the surface or else there are more nerves in that part. The heel is perhaps the least sensitive part of the body, as the nerves do not lie so near the surface there.