Well, the same way wonder BRAS DOMINATED THE '90s, THE Wildly popular spanx shape ware will go down in history as the iconic underthing of this decade. What started as a simple idea, a fresh take on... See More
Well, the same way wonder BRAS DOMINATED THE '90s, THE Wildly popular spanx shape ware will go down in history as the iconic underthing of this decade. What started as a simple idea, a fresh take on the old girdle has now exploded into a billion dollar empire. And the brains of the operation? You may know her as the youngest woman ever to grace the forbes billionaire list. Here's abc's juju chang. Red carpet secret? Spanx. Spanx. Spanx. Reporter: It's the not so secret secret to looking slim You have got to wear spanx. Love them. Reporter: That must-have undergarment. Spanx. Reporter: Celebrities can't stop blabbing about. Spanx. Best inventionmade. Reporter: That word of mouth has paid off. Spanx now sucks in 250 million dollars in annual sales. With not one penny of advertising. I think the magnitude of all of it only hits me in moments. Reporter: The wom behind all those shapely behinds, sara blakely, the inventor and founder of spanx. It's hard for me to absorb. Like, that's a product i created, you know? I created that. Reporter: Spanx is now to shape ware what kleenex is to tissue. A billion dollar household name. What what we hear the most from e-mails and phone calls is confidence. I feel confidence that I don't have normally. Just -- it's a really great feeling. Reporter: Now, she's looking to take that great feeling to a one-stop shopping experience. Come on in, ladies! We're open! Reporter: Blakely is opening spanx first ever stand alone stores. Like this one in king of prussia, p.A. Should victoria's secret be worried? I think so. Reporter: She's now taking on a new set of curves. Everything from yoga pants, swim suits and bras. I'm wearing one of them right now. The bra is so comfortable. The idea was, bring it up to speed and get rid of back fat and have it not dig into their shoulders. So the rest of the bra is made out of panty hose. Hooks, wires, clasps free. Airport friendly. Reporter: Spanx has expanded to more than 200 products, sold in 13 different countries. The simple idea that undergarments can make or break an outfit has turned blakely, at 41, into a forbes cover girl. The world's youngest self-made female billionaire ever. Where do you put that in your head? I think I mostly giggle when I think about it, because it reminds me of those backdrops you would choose at the mall with your friends? You know, like -- Reporter: Look, I'm sexiest woman alive. Right. Reporter: Her success had humble beginnings in clearwater, florida. I was always trying to figure out ways to make money and do little bunesses. I mean, my first memory is drawing pictures with my friend on a rainy day and then selling them door to door. Reporte BUT HER LIFE'S Biggest pivot point was traumatic. She witnessed her best friend get hit and killed by a car. In her grief, she turned to self-help tapes. I memorized all ten tames of wayne dyer's "how to be a no limit person." And that really changed the trajectory of my life. Reporter: The tapes helped her cope with failure and rejection, like when she bombed the exam to get into law school. She spent seven years selling fax machines door to door. Sometimes people would be nasty and rip up my business card in my face. I would sit in my car and cry and put in motivational tape and usually just shift my thinking. Reporter: And then, she had that ah-ha moment that would forever change her life. Sparked by a troublesome pair of cream colored pai ed pants. You could see the panty line every time I put them on, see the cellulite on the back of my thighs. I cut the feet out of control top panty hose one time and I knew that that was the idea that I'd be asking for. Reporter: So, with $5,000 in savings, she started cold calling hosiery mills. One guy called me after i made the rounds, said, I have decided to make your crazy idea. And I found out he ran it by his daughters over dinner and they said, dad, help this girl make it. Reporter: A great idea with a catchy name. But why spanx? I knew instantly it was right, because my product was all about the butt and nobody forgot it. Reporter: What were the runners up? Open toed delilah's. Reporter: What? We would not be sitting here. I know, I realize that. Reporter: As luck would have it, just two months into business, oprah anointed spanx footless panty hose one of her favorite things of 2,000. I love spanx. Reporter: And the rocket had been ignited. Orders all day and all night. Reporter: From a one-woman business to a shape ware empire in just over a decade. I fast forward all these years later and to stand in front of my own store is a real emotional moment. Reporter: Her husband says she's a business titan because she still listens to her intuition. She didn't go to harvard business school. She just trusts her gut. She entered a male-dominated space with no business experience and said, I'm going to go for it. Reporter: Blakely wants to launch a new generation of female entrepreneurs. For now, through a starter program called leg up. And it's my way to pay the oprah moment forward. So, this is a girl named lindsay. She invented a flip-flop that -- Reporter: Oh, cool. The key to blake lip's success is all about taking that lightbulb moment and keeping it glowing. If you are in search of your idea, put on your awareness glasses, look at things differently. I mean, there are million dollars ideas everywhere. Reporter: For "nightline," I'm juju chang in new york.
This transcript has been automatically generated and may not be 100% accurate.