ABC News

First Lady Michelle Obama, with First Dog Bo in tow, surprised tourists in the White House by showing up for an unannounced meet and greet in the Blue Room.

Hundreds of unsuspecting visitors in the tour line got hugs and handshakes - some were brought to tears and others got up out of their wheelchairs to greet Mrs. Obama.

"You're the best!" exclaimed one woman as she struggled to get up out of her wheelchair, determined to stand for the meeting. Mrs. Obama embraced her.

One woman, overcome with emotion, could only repeat, "Oh my God, Oh my God, Oh my God!"

Mrs. Obama attempted to console her: "Don't cry! Don't cry!" she said.

The White House website streamed the 'impromptu' session live and occasionally Mrs. Obama would address the web audience in an aside.

"Are you skipping school?" She asked one girl. When the girl said yes the First Lady looked at the camera and whispered, "We won't tell anybody." And then told her she'd better learn something on the visit.

Dozens of students filed past and the First Lady offered her encouragement.

"Good luck we need some good debaters out there," she told a group of Chicago students in town for a debate competition.

"You gotta be ready to take the mantle," she told Model United Nations students who were visiting the executive mansion.

"Eat your vegetables. Listen to your teachers. Read, please. And listen to your parents!" she told other youngsters.

She didn't talk much about politics during the 80 minute-long session but one young man from Tennessee wearing a Ron Paul t-shirt greeted the First Lady and shook her hand. She didn't comment on the shirt.

Bo got more than his share attention too.

"He's got a great temper," she told some of the Model UN students, as they petted the Portuguese Water Dog "And he's used to it."

This is was the fourth time in as many years that Mrs. Obama has surprised tourists at the White House.

President Obama wasn't home at the time. He's in California raising money for his reelection bid.