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human
ELI5: Blowing out a fire and blowing to start a fire?

You blow out a candle but you can start a fire if you blow onto a small glimming piece of wood. How does that work? Can you also blow out a bigger fire if you blow hard enough and increase a candle flame if you blow a little?
gpt
Fire wants three things to keep burning: fuel, heat and oxygen.

Fuel is already there and will not be affected by your blowing (assuming you're not blowing so hard to blow away the entire candle or the smoldering piece of wood).

The rest depends on how hard you are blowing.

If you are blowing hard enough, your breath (which is considerably colder than the flame) will take away much of the heat of the burning flame (think blowing on a spoonful of hot soup to cool it down). If the flame does not have enough heat left, it will go out.

A smoldering fire is smoldering because it cannot get enough oxygen from surrounding air to burn properly. But if you are blowing into it carefully and softly, your breath brings more oxygen in and this allows the fire to burn more intensely.

(People are often confused about how the air you're blowing can have oxygen - after all you're supposed to be breathing _in_ oxygen and _out_ carbon dioxide. But in reality your lungs cannot extract all of the oxygen you breathe in, so there's plenty of oxygen left in the air you breathe out. And the air you're exhaling also moves more air along with itself as well.)