I had a look around on Google but still confused. Why can I start a fire with a flint rod and a knife. How are the sparks made and what are they.
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Flint or obsidian works well because you can fracture it in such a way to make a very sharp, acute edge. Glass or fine grained ceramic could also be used. Anything. Significantly harder than soft steel which can be given a sharp edge.
Nicking the stone against a steel or soft iron object creates small steel shavings. If the cut is fast enough, then [friction of the shaving against the flank of the cutting edge as it’s peeled off will heat the shaving enough that it can ignite and burn in air.](https://youtu.be/HbL42VqDdCo)
Larger steel objects generally won’t burn because their surface area to volume ratio is too high and combustion isn’t self-sustaining. But items like steel wool which have low weight but high surface area can burn in a self-sustaining way. This is because they have a lot of surface area to pick up oxygen from air.
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