Janet Pribilski, age 12, of Houston, tex. , for her question:

What makes water hard?

All water, of course, is a wet liquid, and liquids become hard only when they freeze solid. But when we speak of hard water, we are sot talking about ice. We mean that our water supply is hard to use for washing hands and faces, for taking baths and doing the laundry. This is because it has dissolved chemicals that refuse to form

Foamy lather with soaps and detergents.

These chemicals are dissolved in fragments too fine to be seen. But they mix with the soap to form grimy scum, and boiling water makes them form a hard crust inside the kettle. They are mostly limy calciums. The water dissolved them from the rocks and river beds on its way to the reservoir. Most hard water runs through limestone regions and gathers its hard chemicals from these soft, chalky minerals.