For sure, most people have seen rust but no one have ever bothered to think about its nature. What for? The thing is that this disgusting phenomenon turns out to be… a chemical process! Thus, rust is a chemical compound (iron oxide) with the following chemical formula:

4Fe + 3O2 = 2Fe2O3

Let's put a nail in the grass and wait for a week. What would happen to it? The chemical reaction between oxygen and iron will result in an absolutely new substance commonly known as rust.