system
Given the following post, respond as if you're commenting on Reddit:
human
What is fire?
gpt
Fundamentally, a fire is a chemical reaction of fuel (charcoal, or gasoline, or methane- whatever you're burning) plus oxygen, which gives off energy in the form of light and heat. The actual flame that you see comes from a combination of two sources.  

The first source of light comes from the actual chemical reaction itself.  As the chemicals react, electrons in the atoms get more energy and transfer to 'excited' states.  They then give off this energy as they go back to their original state.  This gives off light of a single wavelength (the blue at the bottom of gas flames, for example).

Another source of the flame is hot particles of soot and gas.  Just like a metal glows red-hot when heated, the soot and gasses given off by the fire glow red hot from the heat of the chemical reaction.  This is what causes the normal orange glow (of a campfire, for instance).