system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How come blowing on fire sometimes helps it burn (such as when kindling), but other times puts it out (such as candles)?
gpt
Any time a question about fire pops up, it’s useful to start with the fact that combustion needs three things to happen: fuel, oxygen, heat. If any of those is removed or lowered to a certain point, things won’t combust.

Now, if combustion of a fuel releases enough heat to keep the reaction going, and that heat is being released upon more fuel and oxygen, things will keep combusting.

To create a fire, you need vaporized (gaseous) fuel to combust, that’s how you get a coloured gas above your pool of solid or liquid fuel. This means the combustion needs to be hot enough to vaporize whatever fuel it’s burning and keep this vaporized fuel particles hot until they contact oxygen. 

This means that as long as a fire has a steady amount of fuel and oxygen coming in, it’ll be sustained. The moment you take away either the fuel, the oxygen, or the heat, you’ll quench the fire.

Now, when you blow on a lit candle, you’re doing a couple things:

1. You increase the replenishment of oxygen;
2. You blow away the fuel (vaporized candle wax);
3. You cool down the reaction location (gas bubble around the wick).

You can see that because a candle is not burning the wick as its main fuel, but burning the wax, you can quench a candle fire by blowing on it. 

Now, things like kindling are a different story. Kindling is the fuel in this case, and that makes the situation the following when You blow:

1. You add more oxygen;
2. You cool down the zone around the fire. 

Now, because the fuel itself is very, very close to the combustion (solid instead of a gas), the temperature of the fuel can stay high even if the gas bubble around it is colder due to your blowing.

This allows the kindling to become hot enough to vaporize and turn into a fire. You’ll notice that usually the fire only appears after you stop blowing on the kindling, because it’s only then that the vaporized fuel keeps hot enough to combust once it’s released (the gas bubble around the fuel isn’t being cooled down by your blowing anymore)