The Fortune Most Powerful Women Summit kicks off in two weeks with a twist on mentoring: 35 high school seniors will interview certain inspiring women leaders—including fashion entrepreneur Tory Burch, Lululemon (LULU) CEO Christine Day, Gilt Group Chairman Susan Lyne, singer Rosanne Cash—about success and how to get there.

Also in this mix of MPW mentors: Tyra Banks. The supermodel-cum-media entrepreneur created the TV hit, America’s Next Top Model, has released her first novel, Modelland, and is doing a variety of web projects. For a lot of young women, Banks epitomizes power. Last week when I interviewed her for the MPW-Yahoo (YHOO) Power Your Future series, she spoke a simple truth about a lot of powerful women (and many men too): Success, more often than not, is born out of insecurity.

Banks grew from introvert to “mean girl” to “freak girl”—tall and gawky and insecure—by her early teens. “I became a victim of mean girls,” she says, adding, “I became the victim of myself.”

But then on her first day of high school, a girl tapped Tyra on the shoulder and asked, “Are you a model?” That was all it took to give her a little self-esteem. Starting to model in 11th grade, she was college-bound (with acceptances from UCLA, USC, and Loyola Marymount) but got a chance to go to Paris to model. She deferred college and gave herself a year to become a supermodel.

And she did—because, she says, her mother told her that she had to distinguish herself from the competition. Before leaving for Paris, Tyra spent countless hours in the library, studying the top fashion designers and models over the decades. “I have to get a signature walk,” she told herself. Her first year in Paris, Banks broke the record for most fashion shows by a newcomer.

She wrote Modelland in between empire-building and going to Harvard Business School. In the fantastical novel she is speaking to girls and young women as insecure as she used to be. “I want to expand the definition of beauty so more girls can feel beautiful when they look in the mirror,” she says.

Now Banks, 37, wants to have her own kids—and marriage to her longtime boyfriend, investment banker John Utendahl, isn’t essential to her plan, she says. Children would be a huge commitment for a woman who has few hours to spare as it is. But if Banks had one more hour in the day to do something good, how would she spend it? “I think I would get the phone numbers of certain girls who reached out to me and were having some issues with their self-esteem,” she replies. She says she would ask these girls to look in the mirror, find one thing great about themselves—their nose, toes, whatever—and next month, look again. And again.

Real power, says Banks, is “the power to make change, the power to be effective”—and all about passing it on to the girls. “It’s the power to make them feel better,” she adds.

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