Two weeks ago I helped out at "Science Fun Day" at my children's school. I was in charge of the second-grade experiment called "A Shiny New Lincoln". We explored the science behind why copper pennies turn a greenish color, and how to make them look new again.

To begin, I showed the students an example of something else familiar that's made of copper and has become tarnished over time: The Statue of Liberty. Lady Liberty, which was dedicated in 1886, is sheathed in copper that's less than a tenth of an inch thick. Over time, the copper chemically reacts with oxygen in the air, yielding a compound called copper oxide that is greenish in color. It took about twenty years for the Statue to take on the greenish hue that it still has today.

So how did we clean the pennies? Well, copper oxide dissolves in a mixture of a weak acid and table salt. So we used vinegar. You can also clean pennies with lemon or orange juice, because they're weakly acidic too.