Just because he plays a boy genius on FOX's new hit sitcom Malcolm in the Middle and recently made his feature-film debut in My Dog Skip doesn't mean 14-year-old Frankie Muniz hasn't kept his head on straight. Or his hair: Frankie is fond of combing his gelled tresses until each follicle looks perfect. "He's always messing with his hair," says his sister Cristina, 15. "No, I don't," protests her kid brother. "I just like it when it looks nice."

In fact, Muniz (whose mother, Denise, 44, a nurse, and father, Frank, 43, a restaurant manager, are separated) is having a much nicer time of it these days than his TV character, Malcolm, who's embarrassed by both his eccentric family and his 165 IQ, which places him, says Muniz, "among the geeks in school." (His show, meanwhile, placed No. 5 in the ratings its first week.) Frankie himself has been home-schooled by his mother since sixth grade, when he began performing in community theater in his hometown of Wood-Ridge, N.J. From there he graduated to guest shots (Spin City, Sabrina the Teenage Witch) and the lead role in What the Deaf Man Heard, a 1997 TV movie.

When Muniz learned he'd won the Malcolm audition last spring, "I got on the bed and started yelling and jumping," he says. He still, however, has to make his bed. "I really try to keep him grounded," says Denise. "I don't want him to be a brat." Unlike, say, FOX's best-known brat, Bart Simpson. Comparing Bart with Malcolm, "I think we're alike in a lot of ways," says Muniz, "except that I'm a lot smarter." And much better coiffed, dude.