If there's one number most people have memorized, it's their Social Security number. But most Americans don't know the hidden secrets behind those nine digits.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, unravels the mystery of the Social Security number. Dan notes that until recently, the first three digits pointed to the area of the office that issued the number. The middle two digits corresponded to sequential grouping, although the order in which those numbers were assigned included some interesting quirks.

Dan points out that now, though, Social Security numbers are randomized. That is helpful to prevent identity theft, but it presented challenges for financial institutions from Bank of America (NYSE:BAC) on down to the smallest local banks in verifying customer identities. With randomization, much of the mystique of the Social Security number has disappeared.