What even is fire?

Individual oxygen atoms are very very grabby; they're stage-5 clingers on PCP. They're straight-up homewreckers, and they cannot and fucking will not be alone. They need a friend or two, and they will go and rip molecules apart to take them because fuck you.

Now, if there's nothing else available, they'll pair up with another oxygen atom, and form O2, what people normally call oxygen; the stuff you find in the air.

But it's an uneasy alliance, and the bond angles are all wrong so it's kind of spring-loaded.

And the same goes for lots of other molecules - carbon-carbon or carbon-hydrogen bonds ferinstance are also kind of tense and uncomfortable; it takes a surprising amount of energy to snap them into place, like building a tower of interlocking mousetraps.

So smack an O2 at reasonably high speed (or in other words, at a high temperature) at big structure of carbons and hydrogens, and it's fucking chaos.

The oxygen-oxygen bond splits, and the two halves grab the other atoms, ripping the structure apart and releasing all the energy that went into spring-loading those bonds.

The main byproducts are CO2 (a carbon with two oxygens) and H2O (an oxygen with two hydrogens), both of which are very low-energy, strong bonds.

They're both gases, and all that energy leftover is released as heat, which does two things:

• raise the temperature enough to do the same thing with even more O2s, causing a chain reaction
• heat up the released gases (and any bits of random gunk that break off with them) so much that they glow red hot, just like hot iron.

So you get plumes of glowing hot gas-and-particles streaming off the stuff that's burning - and hot air rises, so the plumes point upwards.

But they also cool down quickly in the air, below the glowing-hot point, and that's why flame has a shape: the boundary is how far as they get while still hot enough to glow.

Of course, hydrocarbons and carbohydrates aren't the only things that burn, there's lots of other molecules you can do this to, and the same principle applies. It's just that carbony things tend to burn easily and well, and we're surrounded by the stuff because that's what living things are made of, so that's what you tend to see being on fire the most.