My mother, a meticulous housekeeper, often lectured my father about tracking dirt into the house. One day he came in to find her furiously scrubbing away at a spot on the floor and launching into a lecture.

"I don’t know what you’ve brought in," she said, "but I can’t seem to get this out."

He studied the situation for a moment and, without a word, moved a figurine on the window-sill where the sun was streaming in. The spot immediately disappeared.