Jeremy Pautot, age 7, of Montoursville, Penn., for his question:

WHAT IS WATER?

Water is a chemical compound. It is the most common substance found on earth, and it is also one of the most unusual.

Water is actually made up of tiny particles called molecules. A single drop of water contains many millions of these molecules. And each molecule, in turn, is made up of many millions of smaller particles called atoms.

A water molecule is made up of atoms of hydrogen and oxygen, two of the world's most common elements. Hydrogen and oxygen by themselves are gases. But when two atoms of hydrogen combine with one atom of oxygen, they form a chemical compound called water.

Water can be a solid, a liquid or a gas. No other substance appears in these three different forms within the earth's normal range of temperature.

The molecules that make up water are always morning, and the form water takes depends on how fast they move. The molecules of solid water, which is ice, are far apart and almost motionless. The molecules in liquid water are close together and move about freely. The molecules in water vapor, which is a gas, move about violently and bump into one another.

Most substances contract as they grow colder. But when water reaches 32 degrees Fahrenheit and freezes into ice, it expands. For this reason, ice floats on liquid water. If water contracted upon freezing, any volume of ice would be heavier than an equal volume of liquid water and that block of ice would sink.

If ice sank, the earth would become a lifeless arctic desert. Each winter, more and more ice would pile up on the bottom of lakes, rivers and oceans. Then in summer, the sun's heat wouldn't be able to melt the ice and all water life would die. In time, all the earth's waters would be solid ice. But we don't have to worry about that, because ice definitely floats.

About three fourths of the world's fresh water is frozen in glaciers and icecaps.

Water weighs about 62.4 pounds per cubic foot. Scientists compare the weight of other substances with the weight of water to find the specific gravity of the substance.

Water remains in liquid form when it has a temperature between 32 degrees Fahrenheit, when it becomes ice, and 212 degrees Fahrenheit, its boiling point when it can become vapor.

Boiling water turns into vapor when steam is created. It takes a great deal of heat to produce steam, however. Water reaching the boiling point doesn't immediately turn into steam. First, there is a pause during which the water absorbs additional heat without any rise in the temperature. This heat is called latent heat.

More than five times as much heat is required to turn boiling water into steam as to bring freezing water to a boil. Thus, steam holds a great amount of latent heat energy. Man uses this energy to run machinery.

Water covers 70 percent of the earth's surface. Only about three percent of this water is fresh.

On the average, each person uses about 70 gallons of water each day in his home. On the average, he will take in about 16,000 gallons of water during his lifetime.

Linda Schroeder, age 13, of Dodge City, Kan., for her question:

Yogurt is a smooth, semisolid dairy product made from milk. In the United States and Canada, commercial yogurt is made from cows' milk to which two types of bacteria are added. These bacteria, called lactobacillus bulgaricus and streptococcus lactis, multiply at carefully controlled temperatures and cause the milk to ferment or ripen.

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

The high acid content of yogurt gives the product a sour taste that many people like. Others, however, prefer yogurt that has been sweetened with fruit flavoring.

Yogurt has the same nutritional elements as milk. Some unflavored yogurt contains only a few calories per serving and is popular for low calorie diets.

Some people make yogurt at home, rather than purchase the commercial type. They use commercial yogurt or bacteria from special laboratories to start the fermentation process.

People in Turkey and some of the other countries in the Middle East have been eating yogurt for thousands of years.