After more than two years as America's first lady, Michelle Obama won't say she's hit her stride.

Her performance on a good will mission to Africa, including an emotionally rousing speech about youth leadership and a packed itinerary that rivaled her husband's traveling schedules, said otherwise.

On her second overseas business trip without the president, and to the black motherland, America's first black first lady was warmly received everywhere she went, often with song and to the point of almost being moved to tears.

She spoke passionately about her causes, tickled and danced with some of the youngest Africans, and sat with presidents and first ladies, including Nelson Mandela, South Africa's former president and a hero of the anti-apartheid movement.

She held 20 public events in five days, landed on newspaper front pages and was fashionably dressed, as usual, including outfits with an African connection.

In between all that, Mrs. Obama squeezed in dinner with gal-pal Oprah Winfrey, who was in South Africa for unrelated business.

It was the first lady's biggest moment on the world stage.

She was reluctant to grade herself, telling reporters that it embarrasses her to "talk about my stride and being on my game." But she does realize her power as first lady and says it's a time-stamped opportunity that she doesn't want to waste.

"I have the advantage of really being able to set my own agenda and not having to deal with the day-to-day challenges that ... just keep coming at you," she said, speaking of President Barack Obama. "That's a privilege and there is real opportunity there."

Her signature issue _ both in the states and around the world _ is encouraging young people to become the next generation of leaders and problem-solvers. It's a major reason why she spent a week visiting the model democracies of South Africa and Botswana, her first visits to those countries. In Africa alone, nearly two-thirds of its population is younger than 25.

Mrs. Obama also promoted education and uses the story of her upbringing by working-class parents in Chicago to inspire high school students to dream big.

She lately has taken to arranging for groups of students, particularly those who aren't from the best backgrounds but who have shown academic promise, to spend a day at a top university. She held such as session at the University of Cape Town for 50 South African high school students, following up on one last month at the University of Oxford in London.

"I want to make sure that you all see the promise in yourselves," the first lady told the youngsters. "It's so clear to me and so many others. The challenge is to make sure you see it in yourselves."

Mrs. Obama's message resonated with women in Africa.

"She gives hope not just to women of color, but to women everywhere," said Kiri Maponya, a member of one of Soweto's leading families who now lives in the U.S. The first lady spent Wednesday in Soweto, a black township in Johannesburg that was at the center of the uprisings against apartheid, the now-abolished system of racial separation.

Before the youth leadership speech, Mandela's wife, Graca Machel, gave Mrs. Obama a rousing introduction that nearly moved the first lady to tears.

"We welcome you as a daughter of African heritage and we can call you the queen of our world," Machel said.

Mrs. Obama said she doesn't understand why some things, such as that speech, go really well, and why other things just go OK.

"I just want to be useful," she said.

There's no question that she is useful and will continue to be because, as she often says, there is much more work to do.

For one thing, the presidential campaign season is revving up in the U.S. and her husband wants another four years in office. While she also shies away from the idea that she's the president's "secret weapon," as she came to be known during the 2008 campaign, Mrs. Obama is helping him raise money this coming week at three fundraisers in Boston and Burlington, Vt.

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EDITOR'S NOTE _ Darlene Superville covers Michelle Obama for The Associated Press in Washington.

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Darlene Superville can be reached at www.twitter.com/dsupervilleap