Noah Edwards, age 10, of Champaign, I11., for his question:

HOW LONG DOES IT TAKE A CHICKEN EGG TO HATCH IN AN INCUBATOR?

Most commercial chicken incubators are electrically heated to maintain a constant temperature of from 99.5 degrees Fahrenheit to 100 degrees Fahrenheit. Chicken eggs normally incubate for about 21 days before they hatch.

In the commercial chicken incubator, a system of fans circulate 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 daily, turning all the eggs at the same time. Fresh air brought into the incubator keeps the oxygen level at 21 percent, the normal level of outside air.

Large incubators hold up to 100,000 chicken eggs.

Incubators are also used for human babies. These incubators look like enclosed cribs and not like the ones that hold chicken eggs.

The baby incubator has a transparent cover so that the baby can be seen at all times. The temperature is controlled by a thermostat to keep it even, usually at about 80 degrees Fahrenheit. Oxygen is also supplied. Premature babies are kept in incubators until they can live in outside air.