Rust is the crumbly, brown material which is caused by the chemical reaction of iron, water and oxygen. A scratch on the paintwork of a car allows water to get onto the metal surface where the oxygen in the water chemically reacts with the metal. Eventually the rust blisters, cracks and falls away allowing more water to get in. If rust is left unchecked it eats into the metal, causing holes and weakening it. For example, the Albert Memorial in London has an iron frame covered in lead and marble. Rain got through cracks in the lead and caused the metal underneath to rust. The rust took up more space than the original iron so pushed the lead and marble away.
After viewing, ask: "Is rusting a reversible or an irreversible change?" (It is irreversible - the damaged metal cannot be returned to its former state.) Pupils could collect examples of rusted metal, such as nails, keys or cans, for a classroom display. They could photograph larger rusty items. Children could experiment by rubbing away the rust from a small object like an iron nail, using sandpaper. They could then place their rust-less nails in various conditions to see if they start to corrode again. Pupils might place sample nails in dry, airy, airless and damp conditions. They could check their samples over a period of weeks and keep a photographic record of the results.