The boy watched his father cutting an expensive furnace tile. "Let me try that," he said.

"No," said the father, "a wrong cut will ruin the expensive tile."

"Then I'll watch." He saw his father draw the line in black chalk, place the chisel carefully on the line and repeatedly tap his blows. First lightly, as he circled the tile slowly, and then heavier blows were made.

"It looks to me," said the boy, "like you are getting nowhere. You haven't begun to cut through."

"Watch," said …