Spanx founder Sara Blakely’s $1 billion idea started with just $5,000 in savings and wanting to solve her own problem

New business ideas can come from almost anywhere. Sara Blakely’s bright idea was behind her the whole time—literally.

In 1998, the founder and former CEO of the women’s shapewear brand was working as a fax machine salesperson when she got the inspiration for Spanx. Her white pants had hung in her closet for eight months without wear because she couldn’t get the “smooth look” she wanted. Blakely wanted to wear something that wouldn’t show panty lines and came without the bulk of the classic girdle—so she cut the feet off of control-top pantyhose.

With that, Blakely had come up with the idea that would eventually make her a billionaire.

“I wanted my clothes to fit better, and so my own butt was the inspiration,” Blakely said during Fortune’s 2014 Most Powerful Women Summit. “I might be the only woman in the world grateful to my cellulite.”

Blakely had never taken a business class in her life, nor had she worked in fashion or retail—and she had only $5,000 to her name to invest in the business. A string of hosiery manufacturers turned Blakely’s initial pitch down.

“They would always ask me the same three questions. They would say and you are? Sara Blakely. And you're with? Sara Blakely. And you're financially backed by? Sara Blakely,” she said. “They'd show me the door and say no, thank you.”

What made Blakely’s product take off? Self-assurance.

“I've always trusted my gut through the journey,” Blakely said. “I think it's very important to be authentic.”

Blakely’s big break

While Blakely got a lot of initial “no’s” on her idea that would ultimately transform women’s hosiery as we once knew it, it didn’t stop her from pitching one of the biggest names in fashion: Neiman Marcus. Blakely called her local Neiman Marcus store in Atlanta, where she was living at the time, and they redirected her to the company’s buying office in Dallas.

“I'm Sara Blakely and into the product that's going to change the way your customers wear clothes,” she told them on the phone. Neiman Marcus’ buying office said they’d give Blakely 10 minutes in person to pitch her idea if she’d fly herself there.

Blakely was about five minutes into her pitch when she realized her idea wasn’t totally resonating with the buying representative, so she made a snap decision to model the product. She guided the buying rep to the bathroom, modeled her white pants with and without Spanx on—and in that moment, her concept was received.

“She immediately [said], ‘Oh I get it. It's brilliant—and I'm gonna put it in seven stores,” Blakley said during the Fortune interview. “It was unbelievable.”