What does Brian realize after he throws the hatchet at the porcupine?

Brian realizes that the hatchet can help him make fire.

Fire. The hatchet was the key to it all. When he threw the hatchet at the porcupine in the cave and missed and hit the stone wall it had showered sparks, a golden shower of sparks in the dark, as golden with fire as the sun was now. The hatchet was the answer. That's what his father and Terry had been trying to tell him. Somehow he could get fire from the hatchet. The sparks would make fire.