Here's why:

* Rust is a form of corrosion that happens when iron reacts with oxygen in the presence of water. This reaction is called oxidation.

* Water acts as an electrolyte, meaning it allows the flow of ions, which helps facilitate the chemical reaction between iron and oxygen.

* Even small amounts of water, like humidity in the air, can contribute to rust formation.

While other liquids might have some effect, water is the primary ingredient needed for rust to occur.