system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what makes stainless steel stainless

Just that. Why is some steel stainless and some is not, what’s the difference actually?
gpt
Stainless steel is an alloy of iron with a lot of chromium and nickel added in.

The chromium corrodes quickly in air, to form a layer of chromium oxide. The chromium oxide is very tough and a thin, invisible layer seals the surface of the metal off, so no more air can get in.

Regular iron corrodes to form iron oxide, which is rust. However, rust doesn't seal the surface, so air and water can get penetrate the rust and get to the metal underneath causing further corrosion and more rusting.