The newest member of the Fairfax County police force took up his duties yesterday, moonwalking backward through a crowd of mesmerized first- and second-graders at Clearwater Elementary School, doing a perfect imitation of Michael Jackson singing.

The rookie had just finished a far more prosaic performance -- reviewing the rules for crossing streets safely and staying away from strangers -- through which the group of 250 schoolchildren also sat riveted.

The children had clearly already memorized the rules for crossing the street, so it was not the subject matter that enthralled them but the charismatic performer, the police force's new $15,000 robot.

Known bureaucratically as the Crime Prevention/Traffic Safety Robot and more informally as Officer Robot, the new technology's purpose is to keep the attention of youngsters accustomed to sophisticated entertainments while teaching them the safety rules, according to school and police officials.

The robot will be integrated into the department's existing school safety program.

"This is wonderful," said second-grade teacher Evetta Jones, who sat through yesterday's 30-minute debut performance with her class. "I . . . wish I could use it in the classroom. I'm sure none of my kids were daydreaming during this."

The robot, at 5 feet 4 inches, 180 pounds, is really an overgrown remote-control toy. Manufactured by 21st Century Robotics, an Atlanta company, it is equipped with a cordless microphone, tape player and videotape recorder, and has a television in its chest. It is controlled by a police officer hidden from the children, who acts as the robot's voice, talking through the microphone.

The Fairfax robot -- to be named in a contest among the county's elementary schoolchildren -- is dressed like a Fairfax police officer, with a cap and arm patch. His head is the flashing red light from a squad car and he makes his entrance and exit to the recorded sound of a wailing siren.

Arlington County police also recently purchased a robot from the same company, and it will debut Monday, a police spokesman said.

Officer Chuck Arnone Jr., who asked the robot questions during yesterday's show, said that despite some minor technical problems with the cordless microphone, he was satisfied.

"I was a little worried about whether we were going to be able to control all these kids," he said. "I usually speak to a single classroom, to 100 at the most, but it wasn't a problem at all."

Yesterday's program was leavened with some high-tech humor that had the children howling. "One thing that worries me is strangers," the robot said, turning to stare at the school's principal, L.G. McClung. "There's a stranger right over there," he said.

"That's not a stranger!" scolded Arnone as the children laughed, "that's the principal."

"Oh," responded the abashed robot, "he wasn't programmed into my memory, so he was a stranger." And then the voice of the robot, in this case Officer William Westin, launched into a discussion of strangers and what to do if they bother you.