Here's why:

* A compound is formed when two or more different elements chemically combine in a fixed ratio. Rust is formed when iron (Fe) reacts with oxygen (O) in the presence of water (H₂O).

* The chemical formula for rust is Fe₂O₃, indicating that it contains two iron atoms for every three oxygen atoms. This fixed ratio is a characteristic of compounds.

Therefore, rust is a compound, specifically iron(III) oxide.