American pennies made prior to 1982 were made from copper. When the price of copper started soaring, the government changed the way pennies were made by making a zinc core covered by a copper coating, in 1982. They looked the same, but were cheaper to make since zinc cost less than copper. For this experiment, we need a post 1982 penny.
Zinc is higher on the metal activity series than copper. If HCl makes contact with the zinc, it produces zinc chloride and hydrogen gas. Copper is relatively inactive, and remains behind.
The effect is spectacular. After filing a small nick on the penny so that the zinc is exposed, leave it in 6M HCl overnight. Bubbles from the reaction will be seen almost immediately, but leaving it overnight will result is a hollow copper penny shell, with no zinc left, after rinsing the shell.
The most fun I've ever had for a penny.