why did the back of the penny change
Penny (United States coin) The Lincoln cent is the current one-cent coin of the U.S. It was adopted in 1909 (which would have been Lincoln's 100th birthday), replacing the Indian Head cent. Its reverse was changed in 1959 from a wheat-stalks design to a design which includes the Lincoln Memorial (to commemorate Lincoln's sesquicentennial) and was replaced again in 2009 with four new designs to commemorate Lincoln's bicentennial. There are more one-cent coins produced than any other denomination, which makes the Lincoln cent a familiar item. In its lifespan, this coin has weathered both world wars, one of which temporarily changed its composition as part of the war effort. The obverse is the longest design used for any circulating American currency.
Nickel (United States coin) The silver half dime, equal to five cents, had been issued since the 1790s. The American Civil War caused economic hardship, driving gold and silver from circulation; in response, in place of low-value coins, the government at first issued paper currency. In 1865, Congress abolished the five-cent fractional currency note after Spencer M. Clark, head of the Currency Bureau (today the Bureau of Engraving and Printing), placed his own portrait on the denomination. After the successful introduction of two-cent and three-cent pieces without precious metal, Congress also authorized a five-cent piece consisting of base metal; the Mint began striking this version in 1866.