Here's what I find fascinating about groin-level humor: If you're an adolescent, it works no matter what. But even if your emotional and mental age has advanced beyond the teens, it can still work if it's crafted by someone clever enough to find a new way to get you to laugh at a dick joke.

For those of us who hide our sniggering inner teenager behind chin-stroking meta-analysis, it has become something of a Golden Age of Highbrow Lowbrow. A decade ago, Beavis and Butt-head and South Park mastered the art of smart smut on the small screen before graduating to features; the sentimental shock tactics of the Farrelly brothers felt like a logical extension. Then came the crude-tastic 2005 cinematic double-dip of Wedding Crashers and Judd Apatow's The 40-Year-Old Virgin, and when Apatow's Knocked Up became a hit this summer, it felt official: We had given ourselves social sanction as thinking adults to giggle and think, "Heh, he said 'balls.'"

There is, however, a thin line between smart juvenilia and stuff that's just plain silly -- and Superbad keeps weaving back and forth across that line like Lindsay Lohan at a traffic stop. Produced by Apatow, it marks the first script by Seth Rogen (Apatow's Knocked Up star) and Evan Goldberg (Rogen's childhood friend from Vancouver). So it's not hard to see a bit of autobiography -- or at least wish-fulfillment -- in the story of high school seniors Seth (Jonah Hill) and Evan (Arrested Development's Michael Cera). Soon to be attending separate colleges, the two best friends are looking for one last hurrah by supplying alcohol to an end-of-year party hosted by one of their school's cool girls (Emma Stone). And as they see it, liquor -- to be obtained by their geeky pal Fogell (Christopher Mintz-Plasse) and his brand new fake ID with the nom de stud "McLovin" -- is just the lubricant they need to speed their trip down the road to lost virginity.

Thus begins an odyssey that plays like a cross-breeding of all-night travelogue (think Harold & Kumar Go to White Castle), end-of-school reminiscence (think Dazed & Confused, Can't Hardly Wait) and cherry-popping quest (think American Pie). And true to its roots, Superbad cranks the raunchiness level about as high as it can go in a movie with no actual nudity, including 1) not one, but two montages of penis-based art and 2) using menstrual blood as a punch line (don't say you weren't warned). Many of the finest moments come from Seth's septic tank of a mouth, his verbal assaults on the world seemingly focused entirely on the pubic region. At times, they're breathtakingly hilarious.

At other times -- like some of the rest of Superbad -- they're just a bit too much. One of Seth's finest tirades comes during a home economics class, in which he swears at and insults his teacher -- only to be rewarded with a chance to have a hot babe as his project partner. Superbad loses its way when, in its quest for any potentially shocking gag, it steps into a realm where the characters' behavior isn't even remotely plausible. Sure, you can introduce a pair of dim-witted cops (Rogen and Saturday Night Live's Bill Hader) who like brandishing their guns and getting drunk on the job, but you can't expect the humor to pack the same wallop as material based on awkward exploration of something fundamentally real.

Not that Superbad doesn't also deliver awkwardness in spades. Cera -- easily one of the most instantly endearing young actors since John Cusack -- gets some terrific moments in his flailing attempts to hook up with his dream girl (Martha MacIsaac), including a painfully perfect drunken bedroom scene. Mintz-Plasse is stuck for large chunks of the movie in the sub-plot featuring the two cops, but he nails the glee of a dork getting to play cool for the first time. Don't be surprised if even as you're cringing, you're also laughing a whole lot.

But you might also be laughing a bit less than you'd prefer, especially if you're expecting something as uproarious as The 40-Year-Old Virgin and Knocked Up. Even those of us who groove to four-letter humor can have standards. There's nothing wrong with laughing at gags pitched below the beltline, but there's also nothing wrong with recognizing that not all dick jokes are created equal.

SUPERBAD

*** (three out of four stars)

Starring: Jonah Hill, Michael Cera, Christopher Mintz-Plasse.

Directed by Greg Mottola

Rated R.