system
Given the following post, respond as if you're commenting on Reddit:
human
How does iron exist IN the earth without rusting?

Better yet, how do we have it in our blood without it out oxidizing? There's water and oxygen in our blood.
gpt
Rust is an iron oxide, and iron cannot exist in a pure state in an oxidizing environment--so yes, pure Fe iron is not found in your blood or in the earth. However, iron can form plenty of compounds and complexes in the body and the earth that ARE stable even when water is around to potentially form rust. 

Let's start with blood: iron in red blood cells actually DOES oxidize--but it's not a very strong bond. In your red blood cells you have heme complexes, which consist of an iron atom that is strongly stabilized by a ring of nitrogens. Oxygen can still bind, but it's weak, and the oxygen can dissociate from the iron easily--that's how we transport oxygen in our body. Oxygen associates to and then dissociates from the red blood cells, distributing it throughout our body. The nitrogens stabilize the iron enough that it can "let go" of the oxygen.

In the earth, iron can form a lot of different stable compounds--even ones containing oxygen--that we wouldn't necessarily recognize as "rust". For example, pyrite ("fool's gold") is an iron molecule stabilized by bonding to two sulfur atoms. Both sulfur and oxygen "donate" negative charge to the iron atom, making it stable in the earth. Another example is magnetite--you wouldn't look at it and think "rust", but it's just a pure iron oxide. Magnetite is Fe2O3, "rust" is a hydrated form of FeO2.

On a thermodynamic level, iron can form a lot of different compounds besides rust, and many of them are stable enough that they won't be converted to rust, even if free iron (Fe) would immediately react.