Why does metal rust?
For grown-ups: the science
Iron rusts when it reacts with oxygen and water, making an orange-brown, crumbly material called rust.
Rust is mostly hydrated iron oxides formed by electrochemical oxidation of iron. Water helps move charged particles, and oxygen is reduced; salt can speed corrosion by making the water more conductive. Rust is porous, so it often does not protect the iron underneath.
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Transcript
Look at an old bike left outside. Why does its metal get orange-brown spots?
Some metals, like iron, can meet water and oxygen in the air. Together, they start changing the iron.
The iron joins with oxygen and makes a new crumbly stuff called rust. Rain and damp air help it happen faster.
Rust can spread deeper into iron and make it weaker. Paint or a dry cover can keep water and oxygen away.
So rust happens when iron meets water and oxygen. Keeping iron dry and covered helps protect it.