The metal that will turn into rust if you leave it in the air is iron.

Here's why:

* Rust is a form of iron oxide. It happens when iron reacts with oxygen and water in the presence of air.

* Other metals can corrode but don't form the familiar reddish-brown rust we associate with iron. For example, copper develops a greenish patina, and aluminum forms a thin oxide layer that protects it from further corrosion.