Copper is one of the most common of elements and has been known and used by humans since prehistoric times. Copper is a relatively soft metal in its natural state. By combining copper with another metal, stronger alloys, or combinations, can be made. One of the earliest of these alloys was bronze, created by combining copper and tin. In its natural state, copper has a reddish-brown color, but when copper is exposed to and mixes with oxygen (usually this means with humid air) it changes to a greenish color. This is why the Statue of Liberty, which is made of copper, is green. Copper is used extensively in making electrical wiring because it is a very good and inexpensive conductor of electricity. Pennies have a thin coating of copper but are not solid copper because it would cost more than one cent to pay for the amount of copper needed to make one penny.