Tonya Hermanski, age 13. of Galveston, Texas, for her question:

HOW LONG MUST A CHICKEN EGG STAY IN AN INCUBATOR?

An incubator is an apparatus that maintains conditions favorable for growth and development. Most commercial chicken incubators are electrically heated to maintain a constant temperature of from 99.5 degrees to 100 degrees Fahrenheit. Chicken eggs normally incubate for about 21 days before baby chickens hatch.

A system of fans circulates warm air through the egg chamber and over the eggs. Incubation keeps the relative humidity at about 60 percent to reduce the loss of water content from the eggs.

Mechanical devices turn the eggs several times each day, turning all of them at the same time. Fresh air brought into the incubator keeps the oxygen level at 21 percent, the normal level of outside air.

How many eggs would you say would be found in a single large chicken incubator? The answer to that question: 100,000 eggs.

Special types of incubators are used in hospitals to maintain the lives of newborn or prematurely born babies. Some are also used in laboratories for various research projects. All these incubators differ in design, but their chief function is to provide adequate warmth and ventilation.