First lady greets surprised tourists at the White House

School kids visiting the White House got a brief scare when Michelle Obama popped up and warned them there would be a 50-question test at the end of their tour.

With a straight face she warned the youngsters: "You don't get out of the house unless you pass it." But then she quickly reassured them: "I'm kidding."

The first lady, with dog Bo, set up an informal receiving line Thursday, dispensing hugs and handshakes to startled visitors on the morning tour.

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This is the fourth time she's greeted tour visitors in this manner, according to the first lady's press secretary Hannah August.

August told TODAY.com that Mrs. Obama greeted more than 500 people in a little over an hour.

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When one man told her it was a wonderful home, the first lady agreed, adding jokingly, "We try to keep it up. There's a lot of vacuuming, dusting."

She chatted about fashion, dogs, eating vegetables and more, with surprised visitors.