You're excused if the word "billionaire" causes you to picture an old man slowly stroking a cat and sipping whiskey in the darkness of his garish living room.
But the truth is, you don't have to be a bitter cynic to make money in America. For some, all it takes to join the billionaire club is a fun idea, some luck and a whole lot of hard work. Don't believe us? Just take a look at these success stories: