Most of us feel upset if we miss just one meal, and if we tried to go without food for l2 hours we would really be uncomfortable. But there are some people who seem able to "fast" for very long periods. Various records are claimed for long fast, but in most cases there is no medical proof and so the records are doubtful. One South African woman claimed that she lived for log days on, nothing but water and soda water. There are great differences among living things in the ability to survive without food. For example, a tick, which lives on animals, may survive a whole year. Warm-blooded animals use up their stores of food in the body more quickly. In fact, the smaller and more active the animal, the more quickly it uses up its reserves. A small bird starves to death in about five days, a dog in about twenty. In general, we can say that a warm-blooded creature will die when it has lost about half its normal weight. This matter of weight is important. Man and other creatures live in a state of metabolic equilibrium which means maintaining the body weight once a certain point has been reached. This regulation of body weight is done by thirst, hunger, and appetite. When your blood lacks nutritional materials, this registers in the hunger center of the brain and you feel "hungry". The body is crying out for any kind of fuel (food). And it is our appetite that sees to it that we choose a mixed diet, which is the kind the body needs.