What makes a penny change color?

Pennies become dull or change color with prolonged exposure to air because they are made of copper, which reacts to the presence of oxygen by forming copper oxide. Copper oxide is blue-green and has a common name of "verdigris."

This oxidation affects the surface layers of a penny, leaving the underlying copper largely untouched. According to About.com, the verdigris that has formed on the surface of an old penny can be removed fairly quickly with a short bath in a salt and vinegar solution, which dissolves the copper oxide and restores pennies and other copper items to their original luster.