An alloy is a metal made by melting two or more metals together, or by mixing a metal with another element. Bronze is an alloy of copper and tin. Steel is an alloy of iron and carbon. Brass combines copper and zinc.

Why make alloys instead of using pure metals? Because mixing metals often creates something stronger, more flexible, or more useful than either ingredient alone. Pure gold is too soft for most jewelry, so jewelers create gold alloys by mixing in silver, copper, or other metals to make it more durable. The result is still beautiful but won't bend or scratch as easily.

Ancient metalworkers discovered that adding a little tin to copper created bronze, which was harder and held a sharper edge than pure copper. This discovery was so important that historians named an entire era of human history the Bronze Age.

Today, engineers design alloys for specific purposes. Airplane wings use aluminum alloys that are light but incredibly strong. Some dental fillings use alloys that can be molded when soft but harden into something that lasts for years. Scientists even create new alloys in laboratories, mixing metals in ways nature never did, searching for combinations with exactly the properties they need.