In 1982 the U.S. Mint stopped using 95% copper for pennies, and switched to a cheaper design, made of zinc coated with a thin layer of copper.
In the photo above I have a stack of these pennies, each of which has had a bit of the copper plating filed off. I filed each penny on opposite sides, so the zinc shows through on the top and the bottom edges. If I could remove the zinc, leaving only the paper thin copper, I could string these hollow penny shells on a string to make a necklace.
As it turns out, we can. Many acids will react with zinc, but not with copper. One such acid is hydrochloric acid (known in hardware stores and swimming pool supply houses as muriatic acid). You can make a weak (and safe) form of this by adding salt to vinegar. Or you can buy some of the stronger acid at the hardware store. Of course, the reaction will go much faster with the stronger acid, but if you are willing to wait a day or two, the vinegar and salt should do the trick.
Of course, I chose the faster method. Here you can see hydrogen bubbling up from the pennies as the zinc reacts with the acid.
The acid also cleans off any tarnish from the copper. As it reacts with the tarnish, more hydrogen is released, and you can see that while most of the bubbles are coming from the filed-off places, there is still some coming from the rest of the penny as the tarnish reacts with the acid.
After a few hours, the pennies will float to the top of the acid, and stop bubbling. They float because they are thin copper shells of their former selves, and they are filled with hydrogen.
Now we can add a lot of water to the jar to dilute the acid, and it can be safely poured down the sink. Rinse the pennies well, and then we can string them on a thread or a wire to make a necklace.
A closeup view shows the penny is actually hollow.