It's good sportsmanship to not pick up lost golf balls while they are still rolling.

This quip plays on the gap between the ideal of “sportsmanship” and the self-serving impulses that often surface in competitive leisure. By calling it good manners not to pick up “lost” balls while they are still rolling, the speaker humorously implies that golfers may be too eager to claim another player’s ball the moment it seems headed for trouble—redefining “lost” to suit themselves. The joke depends on timing: a ball still in motion is obviously not yet lost, so the admonition satirizes opportunism disguised as etiquette. In the Twainian vein, it’s a small comic observation that points to a larger truth about human rationalization.