Gresham's law
When a coin is in use as money and the intrinsic value becomes greater than the face value, these coins are in danger of being removed from circulation in large numbers (an expression of Gresham's law). When copper prices skyrocketed in the mid-to-late 1970s, there was a fear that the U.S. one-cent piece might succumb to this fate, leading the Mint to change the composition of the cent in 1982.