Prior to 1982, all pennies were made of solid copper, with the exception of one year.
From 1943 to 1944, America fought WWII with our allies in Europe and Asia.
Because the war effort required a lot of copper, the U.S. Mint decided to stop striking pennies on that material, striking them on steel instead.
As a result, there are still some silver-toned 1943 pennies floating around, which are quite valuable in their own right, relatively speaking. A steel penny in excellent condition might be worth 1000 times its face value — or $10.
Unfortunately, the 1943 steel penny was a less-than-brilliant idea. The steel pennies quickly began to rust.
They also were often mistaken for dimes, and they caused problems with vending machine that used magnets, because the zinc coating made the pennies magnetic, unlike copper coins.
The whole experiment was unsuccessful, and the government quickly attempted to remove all steel pennies from circulation and return to copper. Some steel pennies were coated with a copper veneer as a quick fix.
However, despite the mandate to use only steel in 1943, there were a few mistakes.
At some point, someone struck a very small number of 1943 pennies on copper instead of steel. There are only 3 confirmed in existence.