system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is fire, really? What makes a material flammable at the atomic level?

It can't just be heat, because some materials will just never catch fire. What is it that triggers it, and what is it, other than just energy?
gpt
Fire is hot gasses that result as a product of combustion.  Most commonly in our day to day experience, this combustion is due to reactions with oxygen.

Oxygen is highly reactive.  Given heat and fuel, it will literally rip the fuel molecules apart, generating more heat as it does so.  That makes it self-sustaining; if you continue supplying fuel (and more oxygen), the reaction will continue.

Some materials don't catch fire, because oxygen doesn't react with them.  The form factor of the material matters as well - for instance, a huge cube of zinc wouldn't burn, but powdered zinc would burn.