system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is fire?
gpt
Good question, lots of people know what a combustion reaction is, but not what "fire" is exactly. The reaction itself isn't very important to understand, all you need to know is that when something burns, it releases CO2, which is a gas. Now, imagine a forge. When the metal in a forge gets really hot, it starts to glow a redish tinge. We see this all over the place (electric stove, for example), things heat up and they glow "red hot". This is exactly what's happening during combustion. The CO2 gas floats upwards and is so hot that it glows. This is what we actually see as fire. As it rises and gets further from the reaction it cools and we see other colors before it returns to being transparent. 

Hope that helps!