When Sara Blakely was growing up, her father would often ask her the same question at dinnertime.
"What have you failed at this week?" Blakely recalled in an interview on CNBC's "Squawk Box" n Wednesday. "My dad growing up encouraged me and my brother to fail. The gift he was giving me is that failure is (when you are) not trying versus the outcome. It's really allowed me to be much freer in trying things and spreading my wings in life."
Blakely's embrace of failure has helped make her the youngest self-made female billionaire in America. The 41-year-old Florida native was selling fax machines door-to-door before she came up with the idea for Spanx, the body-shaping undergarments that have become a global sensation.
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Her string of early career failures eventually led Blakely to the Spanx idea. She said wanted to be a lawyer but "basically bombed the LSAT twice," she said. "I ended up at Disney World trying out to be Goofy. They wanted me to be 5' 8", but I was 5' 6". They wanted me to be a chipmunk."
She passed on the chipmunk offer and ended up selling fax machines for seven years. Then she discovered that, as a consumer, "there was a void between the traditional underwear and the heavy-duty girdle." She cut the feet out of control-top pantyhose and made her own modifications to create Spanx.
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It was a long, difficult road from idea to marketplace. She said her own lack of knowledge about retailing and clothing was key.
"What you don't know can become your greatest asset if you'll let it and if you have the confidence to say, I'm going to do it anyway even though I haven't been taught or somebody hasn't shown me the way," she said.
Like many entrepreneurs, Blakely said that not knowing industry practices—and the things that supposedly can't be done—is critical in starting a business.
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"The fact that I had never taken a business class, had no training, didn't know how retail worked," she said. "I wasn't as intimidated as I should have been."
Her rise was filled with little failures—some of them humorous. When she went to London in an early sales trip to promote the product, she was interviewed by the BBC. She described the benefits of Spanx by saying, "It's all about the fanny. It smooths your fanny, lifts and separates your fanny."
Suddenly, the interviewer lost all color in his face.
"I had no idea," Blakely said, "but fanny apparently means vagina in England."
—By CNBC's Robert Frank. Follow him on Twitter: @robtfrank.