Trader Chris Lagana works on the floor of the New York Stock Exchange on Friday as the Dow Jones Industrial Average closed above 50,000 for the first time. (Richard Drew / Associated Press) Round numbers always enchant humans, especially when they're big round numbers. So you'll probably be reading and hearing a lot about how the Dow Jones Industrial Average crossed the 50,000-point threshold Friday for the first time. Actually, "threshold" isn't the right word. The mark's significance is psychological, ...