Bobby Andrews, age 9, of Jamestown, N.Y., for his question:

WHAT IS IN A FIRE EXTINGUISHER?

A fire extinguisher is a metal container filled with water, a liquefied gas or a dry chemical that is used to put out fires. They are easy to operate and often can be used to put out small fire before the flames spread.

Fire prevention experts divide fires into four classes: A, B, C and D.

Class A fires involve such ordinary combustible materials as cloth, paper, rubber or wood. Class B fires involve combustible gases or such combustible liquids as cooking grease, gasoline or oil. Class C fires involve motors, switches or other electrical equipment. Class D fires involve combustible metals.

Class D fires require special extinguishers designed for specific metals. But most other fire extinguishers can be classified by their contents, as one of the three main types: water extinguishers, liquefied gas extinguishers and dry chemical extinguishers.

Water extinguishers are used to fight only class A fires. Water conducts electricity so it must never be used on a fire involving electrical equipment.

A water extinguisher is operated by a valve or a hand pump, depending on the model. Either action shoots the water through a hose that is attached to the container.

Liquefied gas extinguishers may be used on class B and class C fires. There are two main kinds: carbon dioxide extinguishers, which contain gas called carbon dioxide, and Halon extinguishers, which contain another gas. Carbon dioxide has an advantage in that it leaves no residue to be cleaned up after the fire is extinguished.

Each of the liquefied gas extinguishers has gas in liquid form under pressure in the container. The operataor squeezes a handle which converts the liquid to gas.

Dry chemical extinguishers are used on either class B or C fires. Dry chemical extinguishers hold a chemical powder and a gas under pressure. Some multipurpose dry chemical extinguishers can also be used on class A fires.

In a dry chemical fire extinguisher, the gas expels the chemical

after the operator opens a valve or punctures a gas cartridge

attached to the extinguisher. A heat reaction between the expelled chemical and the fire creates carbon dioxide, which puts out the fire.

In a liquefied gas extinguisher, there is no water or powder and for this reason the gas types are the most suitable for class C fires involving computers or other delicate electrical equipment that could be damaged.

In the United States, state and local fire laws require that extinguishers be installed in easily seen places in public buildings. Such buildings include factories, schools, stores and theaters.

School buses and most public vehicles also must be equipped with fire extinguishers.