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The line-dance scene is rarely a good idea. The one in 13 Going on 30, Jennifer Garner’s vagina version of Big, is no exception: At a swank present-day publicity bash for a Manhattan women’s magazine, our not-a-girl-not-yet-a-woman editor heroine, Jenna, tries to get the party started by requesting that the DJ replace his antiseptic house beats with “Thriller.” In tottering heels, a kicky dress, and makeup reminiscent of MJ’s finest era, Jenna rushes the dance floor and earnestly begins performing the clawing, head-dipping, completely awkward monster mash from her fave video. Slowly, others join her, until all the hip revelers are looking just as stupid/happy as can be.

Taken out of context—the clip is being used to promote the film—the scene is nothing less than squirm-inducing. Within the movie, it’s not much better, yet because of the irresistible ebullience of Garner’s Jenna, it’s forgivable. So, in fact, is the film’s It’s a Wonderful Life–esque universe-hopping, as well as the many predictable turns it takes. Ditto for the faintly pedophiliac plot and self-empowering morals about not being a jerk and recognizing your true friends.

Indeed, the success of 13 Going on 30 is due almost entirely to the giddy performances of Garner and co-star Mark Ruffalo, who plays the grown-up version of Matt, Jenna’s girlhood neighbor. The two share a not-so-rosy past: Back in 1987, on her 13th birthday, the eager-to-grow-up Jenna (Christa B. Allen) submits to blackmail in order to get the cool girls, known here as the Six Chicks, to come to her party. When they play a trick on her that lands Jenna in a closet, blindfolded and awaiting her seven minutes of heaven with a crush as the Chicks make for the exits, she blames her nerdy best bud and new closetmate, Matt (Sean Marquette), for scaring them away. Matt, who has just spent three weeks building Jenna her own personal version of a Barbie Dream House complete with “wishing dust,” is naturally crestfallen. Jenna tells him to get lost and goes back to yearning to be “30, flirty, and thriving,” just like the pretty women featured in Poise, her favorite magazine.

Jenna, of course, wakes up the next day blessed with a fab apartment, great bone structure, and the kind of rack that doesn’t hold mix tapes. At this point, of course, 13 Going on 30 stops making much sense. But it also starts being a lot of fun, as Jenna tries to improvise her way through her sudden life as a chic Poise editor.

Garner’s playful, believable turn as the ever-adolescent Jenna is miles away from the action-girl persona that made her a star in Alias. Jenna is a just-right mix of silliness, timidity, and “I never!” bluntness that’s especially refreshing—and surprisingly attractive—coming from a woman who looks as if she’d sooner stick her Prada heel in your eye than giggle. But giggling is something Garner does plenty of here, often accompanied by general freaking out, a combination most entertainingly displayed in a love scene in which Jenna scrunches up both face and body as her boyfriend kisses her ear and then hides behind a pillow as he performs an awful striptease—right down to the tightie-whities—before finally entreating him to “Put it away!”

Ruffalo’s genial, understated Matt is equally—and sorry, there’s no other word for it—adorable. Matt’s skepticism when he one day opens his door to find the grown-up and confused Jenna never quite dissipates, but after he informs her that they’ve barely spoken since that fateful day in the closet, he begins a slow, convincing transformation. Ruffalo’s expressions as Matt watches Jenna act like a nut—smiles at first tentative and cynical, then hopelessly smitten—are aching displays of his recognition of the girl he loved within the woman he loathes.

Musically, the movie is a slightly less obvious version of I Love the 80s!, from the “Head Over Heels” intro to the “Crazy for You”–accompanied closet scene to Jenna’s undying devotion to Rick Springfield and Pat Benatar. (And when was the last time you heard Billy Joel’s “Vienna” played in its entirety in anything?) Good ideas, all, but 13 Going on 30 should really be commended for making appropriate use of yet another bad one: Liz Phair’s unsettling “Why Can’t I,” a dreamy bit of puppy-love wistfulness that starts to seem icky after you discover it’s being sung by a 37-year-old mom. Next to that, a little line-dancing looks positively brilliant.

The cool kids in Mean Girls aren’t the Chicks but the Plastics, and the lead character doesn’t have to skip through time to learn how to deal with them. But otherwise, Saturday Night Live head-writer Tina Fey’s screenwriting debut offers life lessons very much like 13 Going on 30’s, albeit with an evil twist.

Fey based her portrait of socio-academic hell on the ideas of Rosalind Wiseman’s pop-psychology study Queen Bees and Wannabes: Helping Your Daughter Survive Cliques, Gossip, Boyfriends, and Other Realities of Adolescence. The book views teenage girls as a species as mysterious as the apes, suggesting that their society boasts a natural order and ruthlessness that’s more animalistic than human. Helpfully, it also provides advice on overcoming stereotypes and the depression, eating disorders, and backbiting hostilities that accompany them.

The movie’s sorta-anthropological basis, however, doesn’t much help to distinguish it from your usual tired teen comedy. The story of Cady (Freaky Friday’s Lindsay Lohan) is fairly typical: The 15-year-old is a new student at a Chicago high school and is trying to figure out which group to hang with. And Cady is really, really new, having previously been home-schooled by her parents in Africa.

Cady is first taken in by the fringe, consisting of the punky maybe-lesbian Janis Ian (Lizzy Caplan) and tubby gay boy Damian (Daniel Franzese). They warn her of the Plastics, and when the pretty newcomer is lured by the siren song of the ringleader, Regina (Rachel McAdams), Janis and Damian encourage her to infiltrate and sabotage.

Mean Girls is saved from total ho-humness by Fey’s script, which contains some on-target parodies of the ridiculous stuff of high school: “Don’t have sex! Because you will get pregnant—and die! Here’s some condoms.” Director Mark S. Waters, who also worked with Lohan in Freaky Friday, is a little less original this time around, though, including scenes such as the slo-mo cool-girl parade down the hall and lighting the Plastics glamorously while keeping Cady & Co. unenhanced and “real.”

Until about the film’s halfway mark, that is, at which point Cady stops being a natural teen and starts walking around with her boobs pushed up to her neck. The worst sin committed by Mean Girls is not its pratfalls or its fart jokes—it’s the movie’s blatant hypocrisy. Even when Cady does get the message that it’s better to be faithful to your real friends than to go around hatin’ in the name of popularity, her shirt is about two sizes too small and her long hair is flowing. Cleavage is rampant, and a talent-show performance of “Jingle Bell Rock” is nearly worthy of the adult section. There’s even a shot of Regina’s kid sister practicing her, uh, milkshake in front of an MTV-blaring television.

So…whether you’re a mean girl or a good girl, it doesn’t hurt to be sexed-up and camera-ready? Somehow, I doubt that’s what Wiseman had in mind. CP