[ELI5] How come air can extinguish a fire (e.g. blowing a candle) but also ignite it (e.g. blowing a spark)?
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You remember the heat+oxygen+fuel fire triangle from school, right?
Blowing out a candle does three things:
1. It blows away the vaporised wax which is what acts as the fuel for the flame. It’s akin to taking out wood away from a campfire
2. You suddenly blow away hot air and introduce colder one in it’s place. Because enthropy causes every system to roughly equalize the heat from the flame suddenly travels into the colder air, equalizing the entire system for a brief moment lowering the heat of the flame for a short while(until more enrgy is released from the fuel)
3. You blow out the air from your own lungs, this air is generally more dense in carbon dioxide which also stifles the flame
You are reducing the balance of three main components needed for fire to exists, thus making the candle flame unsustainable with what it has in that brief moment and causing it to go out.
Meanwhile blowing a spark helps with igniting it because that spark doesn’t have all those 3 components in the first place – it has the necessary fuel and heat, but it lack the oxygen, and by blowing air on it you are forcefully moving oxygen molecules into the places near the actual heat source from which it was already used up, allowing you to sustain the flame
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