Think of the greatest structures of the 19th centuryâ??the Eiffel Tower, the Capitol, the Statue of Liberty and you'll be thinking of iron. The fourth most common element in Earth's crust, iron has been in widespread use now for about 6000 years. Hugely versatile, and one of the strongest and cheapest metals, it became an important building block of the Industrial Revolution, but it's also an essential element in plant and animal life. Combined with varying (but tiny) amounts of carbon, iron makes a much stronger material called steel, used in a huge range of human-made objects, from cutlery to warships, skyscrapers, and space rockets. Let's take a closer look at these two superb materials and find out what makes them so popular