Read the excerpt from "How the Whale Got His Throat.” For the Mariner he was also an Hi-ber-ni-an. And he stepped out on the shingle, and went home to his mother, who had given him leave to trail his toes in the water; and he married and lived happily ever afterward. So did the Whale. But from that day on, the grating in his throat, which he could neither cough up nor swallow down, prevented him eating anything except very, very small fish; and that is the reason why whales nowadays never eat men or boys or little girls.

What is the central idea of this passage?