Amy Cooper, age 12, of Butte, Mont., for her question:

WHY DO WE FEEL HUNGER?

In man and animals with backbones, chemicals in the fluid surrounding the brain and spinal cord probably play an important role in regulating the sensation of hunger. If some of this fluid is taken from a hungry animal and injected into an animal that has just finished stuffing itself, that animal will start to eat again.

The amount of glucose or sugar in the blood also affects hunger. People may become hungry if they have less than a normal amount of glucose in their blood.

. The most familiar hunger pains are stomach cramps, which are produced by strong contractions of the stomach muscles. Appetite is related to hunger, but it is merely a desire for food that we like.

Nerves called internal chemoreceptors in the brain, kidneys, digestive system and elsewhere in the body detect the need for such things as vitamins, minerals and amino acids when they are lacking. This action may lead to eating the needed foods.

In man, hunger and food intake are also affected by social habits. We eat at particular times and we usually eat only certain foods.