"They might have the wedge longer then," said Rollo, "and then it would be just as tapering, and yet it would be a great deal broader at the back, because the back would be farther off." "That would make the wedge a great deal too heavy. It would not drive." "Why, yes, it would," said Rollo. "No, it would not," said the boy. "It would be just like a shoemaker's lap-stone; pounding it would hardly move it." Rollo did not understand what the boy meant by what he said about the shoemaker's lap-stone; so he paused a moment, and presently he said, "I don't think it would make any difference, if it was heavy. And, besides, it might be made of wood, and that wouldn't be heavy." "O, wood wouldn't do," said the boy. Now it happened that while they had been talking, the boy had gone on driving in his wooden wedge into the cleft that the iron one had made, and it had been gradually splitting the log open more and more. So that just as the boy was saying that "a wooden wedge wouldn't do," Rollo was actually seeing with his own eyes that it _would_ do; for at that moment the boy gave the last blow, and the halves of the log came apart and fell over, one to one side, and the other to the other. "Why, there," said Rollo, "you have split the log open with a wooden wedge." "O, that is because I had an iron one in first," said the boy. "What difference does that make?" said Rollo. "A great deal of difference," said the boy. "But _what_ difference?" persisted Rollo. "I don't know exactly what difference," said the boy; "only I know you can't do any thing with a wooden wedge until you have first opened a seam with an iron one." Rollo was confident that it could not possibly make any difference whether a wooden wedge was used first or last. The boy was sure that it did, though he could not tell why. Finally, they determined to try it; so the boy struck his axe into the end of the next log, and then attempted to drive in his wooden wedge. But he did not succeed at all. The wedge would not stay. Rollo told him that he did not strike hard enough. Then he struck harder, but it did no good. The wedge dropped out the moment he let go of it, and on taking it up, they found that the edge of it was bruised and battered; so that even Rollo gave up all hopes of making it enter. "Ah!" said the boy, taking up the wedge, and looking at it, "now I know what the reason is. It is the edge." "Where?" said Rollo. "Let me see." "Why, when there is no crack," said the boy, "you see the edge of the wedge comes against the solid wood, and when I drive, it only bruises and batters it; but the iron is hard, and goes in. But then, when a crack is made, the wedge can go in easily; for the edge does not touch; then only the sides rub against the wood." "How?" said Rollo. "I don't understand."