system
Given the following post, respond as if you're commenting on Reddit:
human
Why does fire go up?

I was holding a torch the other day and when i moved it towards the ground I noticed the flame got bigger, and when i lifted it up toward the air, the flame got smaller (while moving).  I deduced that this was occuring because the flame naturally goes up, so by moving down I'm injecting more Oxygen into it...but then I had to wonder, why does fire go "up"? Seems like it should form into an elongated ball around the ignition point.
gpt
The visible red-orange-white flame of a fire is fine solid/liquid particles of half-burnt fuel (soot) that are so hot that they are glowing (incandescence). This particles are small enough that they are carried about by moving air. The air in a flame is heated by the reaction and becomes less dense. Gravity pulls harder on the surrounding colder/desner air than on the hotter/lighter air in the flame. As a result, the cold air rushes down and under, forcing the hot air to go up. The glowing soot particles get carried by this updraft of hot air and you see a flame billowing skywards. 

In summary, flames go up because gravity pulls colder air down.