JoinScience 101: What causes things to rust?
The simple answer is that things rust when you leave them out in the rain. But maybe you want a more thorough answer. Rust is a form of corrosion, which is broadly defined as the wearing away of materials due to chemical reactions. So corrosion applies to materials other than metals. Metal corrosion is the one we commonly think of, though, such as tarnish on silver and that filmy covering (aluminum oxide) that forms on aluminum.