Diana Navarrete, age 8, of Visalia, Calif., for her question:

WHAT MAKES SILVER TURN DARK?

Silver is softer than copper but harder than gold. It can be hammered into sheets so thin that it takes 100,000 of them to make a one inch stack. The sheets are so thin that light can shine through them. Silver can also be drawn out into wire threads finer than human hair.

Silver does not change when it meets moisture, dryness, alkalis or vegetable acids. But sulfur, or air that contains sulfur, can cause silver to turn black. For this reason silver tarnishes very quickly in places where coal gas, which contains sulfur, can be found.

Tarnish is the word used to describe the condition when the metallic or mineral surface of an object loses its luster and becomes dull or discolored.

In silver, a special coating can often be applied to an object to prevent tarnishing.