system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is fire?

I think I know fire is gases, but physically what in the heck is fire? You can't physically grab it, but you can certainly feel it. What in the heck is it?
gpt
Fire is the rapid oxidation of the molecules in the fuel source (i.e. logs in a fireplace).  It is an exothermic reaction which is where the heat comes from.  The energy generated from the oxidation of the atoms excites the electrons in the combustible material.  When the excited electrons fall back down to their ground state, they release photons which is the source of the light that you see (i.e. flames).