Eddie Woodward, age 14, of Phoenix, Ariz., for his question:

WHAT CAUSES A COUGH?

One of the most common symptoms of some problem in the respiratory system is a cough. It is caused by a simple reflex: usually an automatic effort of the body produces the action.

You cough to rid the lungs, air passages or upper food passages of material that should not be there. You also cough when you have a cold and need to spit out mucus or phlegm that has built up in your respiratory system. You'll also cough if you happen to inhale dust, fumes or anything to which you personally may be particularly sensitive or allergic.

The mechanics of a cough are rather simple. First you suck in air and momentarily hold it in your lungs until you build up pressure. Then you suddenly open the valve like glottis in the larynx and force the air to rush out with tremendous force.