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They don`t act like kids, these two 13-year-old runt rappers who operate under the moniker Kris Kross.

Sitting in a conference room at Columbia Records, Chris Smith and Chris Kelly don`t fidget or blather. Instead, they maturely and matter-of-factly answer questions about how a pair of pipsqueaks from Atlanta just managed to earn the fastest-breaking pop single by a new act in 15 years.

Three weeks ago, their maddeningly insistent rap ”Jump” had bolted from No. 61 all the way up to No. 12 on Billboard`s Hot 100 pop chart, going gold in seven days. It has now reached No. 1.

In one fell swoop, this duo`s singsongy, skip-rope anthem has turned them into the most instantly famous rap act since Naughty by Nature.

The duo isn`t greatly surprised.

”We knew it would be big,” Chris Kelly says, blithely. ”We didn`t know it would be this big.”

Then again, the kids` success hardly comes without precedent. Last year, the assemblage of romper room rappers known as Another Bad Creation broke big, under the careful guidance of Michael Bivens (of Bell Biv Devoe).

Likewise, Kris Kross is the brainchild of producer Jermaine Dupri, who at 19 has overseen work by Damian Dame, Silk Tymes Leather and the suddenly hot TLC (”Ain`t Too Proud 2 Beg”).

For the two Chrises, meeting Dupri was a matter of being in the right mall at the right time.

”I saw them walking around and they just looked like a group,” says Dupri. ”I wasn`t interested in them as performers at first. It was their look and the way they were together.”

Dupri later thought about using them as dancers, ”but we said we rap,”

Smith explains. Dupri was impressed enough with their potential to fashion a whole persona and style around them. He also writes all their material.

First came the clothes: Expanding on the idea of turning baseball caps around, Dupri told the kids to wear everything backwards.

Walking around the malls with that wrong-way look didn`t go over big at first.

”Kids would say, `How come you got your pants on backwards?` ” Smith explains. ”Now other kids are doing it.”

Dupri also wanted to break new ground in what kids could rap about. Unlike Another Bad Creation, which sticks to the traumas of swing sets and seesaws, Kris Kross is presented as little boyz in the hood, chronicling gang violence and drug problems in between the expected cautionary tales about missing the school bus.

”I wanted it to be something kids my age would like as well,” Dupri explains.