Melanie Takata, age 11, of Visalia, Calif., for her question:

WHY DOES COPPER TURN GREEN?

Copper is actually very resistant to corrosion. It will not rust. In damp air, however, copper will turn from a reddish orange to a reddish brown color and after long exposure it will turn green with a film called patina. The patina is part of the chemical reaction of the metal, and it helps to protect the copper against further corrosion.

Copper is one of the best known conductors of electricity. While silver may actually be a bit better, it is too expensive for common use.

We've been using copper for more than 5,000 years. Today it is used to make everything from rain gutters for houses to electronic guidance systems for space rockets. Perhaps the most famous copper object found today is the penny.