Valerie McCaffery, age 15, of Billings, Mont., for her question:

WHAT IS THE NORMAL BODY TEMPERATURE?

Normal body temperature of a healthy, resting adult human being is 98.6 degrees Fahrenheit. Doctors say that a temperature within one degree Fahrenheit of this figure can also be considered normal.

A temperature higher than 98.6 degrees Fahrenheit may indicate a fever and a temperature lower than this may be a sign of old age or of certain illnesses.

Body temperature is a measurement of the heat in an animal's body. The body of an animal generates heat by burning food. But the animal also loses heat to    or gains heat from    its environment.

The body temperature of warm blooded animals, which takes in birds and mammals including man, almost always remains fairly constant, regardless of the temperature of their environment. The body of a warm blooded animal balances the amount of heat it exchanges with the environment with the amount it produces by burning food. All other animals are cold blooded animals.

Cold blooded animals cannot balance a heat exchange accurately. As a result, their body temperatures tend to vary according to the temperature of their environment.

Warm blooded animals make various physical and behavioral adjustments to regulate their heat exchange with the environment. In cold surroundings, they increase the production of body heat and decrease the amount of heat lost to the environment. In hot surroundings, they do just the opposite.

A part of the brain called the hypothalmus controls the body's heat adjustments. Certain nerves in the skin and deep within the body send messages to the hypothalamus. The hypothalamus compares the temperatures of these areas with that of the brain.It then triggers the necessary responses by various nerves and glands to maintain a normal body temperature.

Each species of warm blooded animals has its own normal body temperature. Each species also functions best when the temperature of its surroundings remains within a certain range. This range varies greatly from species to species, depending on such factors as the thickness of fur and the rate at which its body burns food.

Even with the various controls, the body temperature of a warm blooded animal does not remain entirely constant. It changes slightly during the day.

In a healthy human being, as an example, the body temperature is lowest in the morning and then rises until late afternoon. It fails again during sleep.

Strenuous activity can rise the body temperature. Also, in cold surroundings, the temperature of the akin and limbs may drop far below the temperature within the body.

Cold blooded animals lack the precise temperature regulation abilities that characterize warm blooded creatures.