When you strike a spark, you are shaving tiny pieces off the steel with the sharp edge of the flint. The pieces become incandescent from the friction. The sparks come from the steel, not the flint. The harder the steel, the smaller, and hotter, the pieces will be. The sharper the flint, the more sparks you will get. Strike down the steel with the flint at a shallow angle. Point the steel into your tinderbox to direct the sparks into your tinder. Learn to use short, choppy strokes so you can hold the steel close to the tinder without hitting the tinder with the flint. Keep your fingers back from the edge of the steel so you don't cut yourself with the sharp flint. When your flint becomes dull, chip it back to a new, sharp edge. Keeping the edge of the flint at a very shallow angle to the steel will increase the useful life of the edge.