Eddie Kushman, age 9, of Carson City, Nev., for his question:

WHAT CAUSES YAWNING?

Yawning is an involuntary reflex. It is the act of opening the mouth wide, or gaping. The usual yawn is due to drowsiness or fatigue. Yawning is a sign that the body needs sleep.

After you have started a yawn, it is almost impossible to stop it. You can try to hold your mouth closed but the yawning muscles still contract.

You yawn when oxygen is slowly cut off and when your muscles are thoroughly relaxed. If you yawn often, you are probably not getting enough oxygen. It may be that you need better ventilation or exercise.

You'll generally stop yawning if you drink a beverage or bathe your face with cold water. Scientists are not sure what part of the nervous system controls yawning, but it may be the mesencephalon or mid brain in the brain.

One purpose of yawning may be to awaken a person by stretching the muscles and helping the blood to circulate.