What causes iron to rust?

Answer: Air and water

Air and water ✓ — Correct. Oxygen in the air combines with iron, and water helps the reaction happen. Iron loses electrons to oxygen, forming iron oxide, which is rust. Without both oxygen and some water, the iron stays largely unchanged.

Temperature changes — Wrong. Temperature can speed up or slow down rusting, but heat or cold does not turn iron into rust by itself. Rust is a chemical reaction between iron, oxygen, and water, not a physical change caused by the weather.

Dirt and grime — Wrong. Dirt and grime do not react with iron to make rust. They can trap water and air against the metal, which indirectly encourages rusting, but they are not the real chemical cause.