A Tinder Box contains very combustible fibers which will smolder when a steel spark is struck on it by a piece of Flint (Both shown above).
The saying, "Keep your powder dry" applies equally to Tinder !
This same principle is used in Flintlock Rifles where a piece of Flint is released by a squeeze of the trigger, to strike a piece of Steel and produce a spark which explodes the gunpowder and propels the bullet.
This principle is also used in cigarette lighters, where a steel thumbwheel is rotated against a piece of flint, to project a spark onto a wick saturated with volatile lighter fluid.
It is the Steel that makes the sparks, the same as grinding a hatchet on a grindstone. The Flint, being a distant cousin of Diamond, is hard and likewise removes particles of steel. It must be properly hardened to spark. Soft steel makes few, if any, sparks.
The Tinder may consist of anything combustible, such as grass, hair, cloth, and pithy wood. Sometimes it is prepared by baking it in the absence of oxygen. After 20 minutes in a sealed tin can placed in a campfire, moisture is driven from the Tinder, and it is so charred that it will ignite readily.
A slow steady breeze from puckered lips will cause the spark to glow and ignite the Tinder, but wind is the greatest enemy of successful fire starting using this method.
The Tinder is consumed so quickly by fire that often a candle may complete the kit, and be the actual flame used to light the campfire. Then a new batch of Tinder could be baked to start the next campfire.