Russ Bailey, age 9, of Colonial Heights, Va., for his question:

WHAT IS HELIUM?

Helium is a lightweight gas and chemical element. Only hydrogen weighs less than helium. It is called an inert or noble gas because it does not combine with other elements.

The sun and many of the stars are made mostly of helium and hydrogen. Helium, the scientists tell us, makes up about 23 percent of the matter in the visible universe.

On earth, helium is found in natural gas deposits and in the atmosphere. About five parts of helium per million parts of air are found in the atmosphere.

Helium is used in scientific balloons. It is safer than hydrogen because it will not burn, as hydrogen will. The gas is also used in heliarc welding. The government uses helium to help maintain the proper pressures in rockets.