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

As a fire is still burning, what is the visible part of the fire made out of?
gpt
Hot gas. Fire happens when something reacts chemically with oxygen. This reaction releases heat; this heat raises the temperature of the reaction products. Some of the reaction products are gases like carbon monoxide or carbon dioxide. When these gases get hot, they glow, just like a hunk of steel glows when it gets hot. So the visible flame you see when something burns is gas released by the chemical reaction and heated to the point where it emits light.