Sara Burnett, age 8, of Charlotte, N.C., for her question:

WHAT IS USED TO MAKE SOAP?

Chief ingredients of soap are fats and chemicals called alkalis.

Either animal fats or such vegetable oils as coconut oil and olive oil can be used. Most soapmakers use sodium hydroxide as the alkali.

A continuous processing method today can make as much soap in a few hours as the old kettle method made in several days. A stainless steel tube called a hydrolyzer is used. The tube is about three feet in diameter and about 80 feet in height.

Water under pressure and heated to about 500 degrees Fahrenheit is pumped into the top of the hydrolyzer. At the same time, a machine pumps in hot fat at the bottom. The fat splits into fatty acids and glycerol and rises to the top where it is removed, purified and mixed with alkali to make soap. The soap is then mixed with other ingredients in a crutcher to make bars.