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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Your “flint rod” is not flint the mineral. It is an alloy of iron, cerium, and other various rare earth metals (lanthanides). When you scrape away tiny flakes of it with your knife the friction heats them and they ignite in the oxygen of the atmosphere. The cerium etc. lowers the ignition temperature of the flakes, and the iron provides fuel, and structural integrity to the rod.
It doesn’t have to be flint, just any rock harder than 6 on the mohrs scale. Basically, as you strike the stone with the steel, a small piece of metal will be shaved off. If the piece of metal is small enough, it will oxidize (react with oxygen) faster than it can dissipate heat, creating a spark
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.
Steel is strong (it can hold lots of weight), but it isn’t necessarily very hard (it can be scratched easily). Any rock harder than steel can potentially be used to create sparks.
What happens is that when you strike the rock and steel, the harder rock will cause tiny flakes of steel to chip off. Because steel is strong, the bonds holding it together have lots of energy stored in them. When the flake is chipped off, this energy turns into heat, causing the little flake of steel to get very hot. The flakes are so hot that they glow red or yellow, and these flakes are the “sparks” we see coming off the flint and steel. If a hot flake of steel falls onto something flammable, it can start a fire.
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