Carlin Otto Jr., age 10, of Longview, Wash., for his question:

HOW IS YOGURT MADE?

Yogurt is a smooth, semisolid dairy product that is made from milk of cows, buffaloes, goats and other cud chewing animals. In North America, commercial yogurt is made from cows' milk.

Yogurt makers add two types of bacteria to milk to make yogurt. These bacteria, called lactobacillus bulgaricus and streptococcus lactis, multiply at carefully controlled temperatures and cause milk to ferment or ripen.

During fermentation, the bacteria change milk sugar into a syrupy substance called lactic acid. This acid causes the milk to thicken into yogurt.

Some people make yogurt at home. They use commercial yogurt or bacteria from special laboratories to start the fermentation process. Yogurt making machines are available for home use.

The high acid content of yogurt gives the product a slightly sour taste that many people enjoy. Yogurt has the same nutritional elements found in milk.