You know it, Jim. But most people do not know it. Since 1982 our US 'copper' pennies have been minted out of zinc, not copper. Why? Because the value of one copper cent is more than one cent.
Some 1982-dated pennies, and *all* post-1982 pennies are made of zinc with a thin coating of copper added.
Drop a pre-82 cent on a hard surface. You hear a pleasant ping. Drop a post-82 cent on a hard surface. You hear a dull thud. Get the difference?!
Sometimes unplated cents are produced by the mint by mistake. These minting errors usually look like the cent in our picture. They are genuine mint errors and sell for $1 US to $10 US dollars or more, depending on overall appeal.
But there are plenty of post-minting ways to get the copper plating off a zinc penny. An acid bath is one. There are several others, including science experiments by kids in school. Cents with plating artificially removed after minting are worth zero.
Don't get these zinc coins confused with 1943 *steel* Lincoln cents. Steelies are appraised on this page.