Inheritance, luck and stock market gambles all play a part — but for most wealthy Americans it's a different story.

Rafael Badziag interviewed 21 billionaires for his book, "The Billion Dollar Secret," and they all told him how they made their first $1 million.

A lot of people set the goal of saving $1 million for retirement because it's a nice round number. But before you get your mind set on aiming for a $1 million nest egg, you may want to think about ...