A Rubicon or just a Rubik’s Cube? The Dow Jones Industrial Average this week crossed the 10,000 mark for the 48th time, to much excitement. The puzzle is less what any particular point on a narrow and flawed index means, but rather why such round numbers hold a fascination in the first place.

Perhaps it is because after the first 1999 foray into five figures, the Dow has seemed determined to stay nearby. It has averaged 10,501 ever since, even while swinging between a low near 7,000 and the 14,000 high-water mark.