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

Does fire spread, or does the heat from the previous flame match the autoignition temperature and make more flames?
gpt
It's mostly heat transfer, but also diffusion of the intermediate combustion products.  Even with really simple fuels like methane, you have tons of different (some highly reactive) molecules/ions/radicals produced and consumed within the flame.  So if you held a flame to something vs. blowing hot air of the same temperature on it, the flame could actually do a better job of setting that thing on fire.  Search "[fuel of your choice] detailed reaction mechanism" to tumble down that rabbit hole.

There is also the concept of flame speed.  Gas phase products get hot and therefore expand, and that pushes them away from the flame (or pushes the flame away, depending on your reference frame).  So a flame will just automatically climb upstream at a speed determined by its chemistry and heat transfer to surroundings.  Searching "laminar flame speed" will get you started on that...