Here's why:

* Rust is a form of iron oxide. It's what happens when iron reacts with oxygen in the presence of water.

* The chemical reaction: Iron (Fe) + Oxygen (O2) + Water (H2O) → Iron Oxide (Fe2O3) (rust)

While water isn't a gas, it's crucial for the reaction to occur. The water acts as an electrolyte, allowing the oxygen to more easily react with the iron.