system
Given the following post, respond as if you're commenting on Reddit:
human
What actually IS a flame?

Why does fire create a visible flame, why does it exist and what is it made of?
gpt
A flame from something like a candle is visible because it's filled with three types of atoms that are going through changes. The paraffin in the candle - wax - is a long chain of hydrogen and carbon that's linked together. Add heat, and that long chain first melts (to form liquid wax) then evaporates (to form that puff of "steam" on the end of a just-blown-out candle wick), and then the links start shattering and it breaks down into carbon and hydrogen atoms, or small groups of the same three ingredient atoms. (Wood does the same thing, actually, except when it breaks down there's more leftover carbon and other stuff, and because it doesn't melt, the process isn't usually as clean).

For a brief moment, those carbon atoms - soot, really, that got broken out of the wax rise due to the heat - gets super-hot and give off yellowish light as a result. This is called "black body radiation". Many of these carbon atoms react with oxygen to form CO2 (carbon dioxide) and give off heat, but before that happens they'll glow. (The hydrogen in there also gives off a little visible light when it burns, but not very much compared to the glowing unburned carbon atoms.

If you slowly burn something like propane on a low-set stove, you'll get a different result. Get enough heat and oxygen in there and smaller molecules start "burning" (reacting with oxygen) before they give off much glow, and instead only produce a light blue light  That's why a propane stove's flame set to "low" is blue. But push too much fuel through and you'll get lots of broken-out carbon atoms (which add a yellow color when the carbon heats up, and leave soot behind) instead of a nice clean burn. So cook lots of stuff on "high" on a small stove and you get yellow flame and pots with black bottoms. And burning charcoal - burning carbon - is red, but not BRIGHT red - because it's not burning fast and heating up carbon atoms that have been released by its heat and are floating above it.

So a flame is an area above a fueled heat source such as a campfire or candle or matchstick, that gets hot enough to make fuel to turn to gases or broken-down molecules. Those molecules  give off a little visible light as they burn, but depending on the type of flame, the "dust" that's in there gives off a lot of visible light due to the heat, and the chemical reaction gives off a little.