FOR THE past year, American households have reverberated to the following dialogue: "Dad, can I watch South Park tonight?" "Over my dead body, kid." "But Dad, it's a cartoon." "I know that, kid. But it's not a cartoon for kids. It's a cartoon about kids. Kids ain't s'posed to watch it."

South Park, which is coming to Channel 4 next month, is set among the snowy peaks of rural Colorado, the world capital of UFO sightings. The grotesque nine-year-olds, muffled in parkas and bobble hats, waiting at the bus stop in the first episode could be mistaken for aliens themselves. They'd be of the furry, malevolent half-pint gremlin variety, except that they are no more than visitations from your own infantile psyche.

There's Cartman, the fat bolshy Cheesy Puff-addict who is troubled by fiery flatulence when aliens plant an anal probe inside him. Stan is the sporty one who vomits green bile every time his classroom sweetheart talks to him. Kyle is a bright, Jewish neurotic who has an imaginary singing, dancing friend called Christmas Poo. And Kenny, the mumbling runt of the group who dies a horrible death in every episode.

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If the child is the father to the man, then check out South Park's adults. The kids' disturbed teacher, Mr Garrison, keeps order through his glove puppet, Mr Hat. Stan's uncle is a gun fanatic, whose Viet-vet sidekick speaks through a voicebox. Cartman's mom is alleged by his friends to pose for Crack Whore magazine.

South Park was launched in the United States last August. It's not the first animated entertainment to look beyond the world of adults in search of human foibles. In their different ways, Beavis and Butthead, The Simpsons and King of the Hill have already acknowledged that adolescence is a key breeding ground for dysfunction, neurosis and sociopathic tendencies. But none is so uncompromisingly frank that the junior generation portrayed is actually discouraged from watching. South Park comes with a health warning: "The following program contains coarse language and, due to its content, should not be viewed by anyone."

The makers of South Park are a couple of guys in their late 20s from Colorado who met at university in Boulder in the early 1990s. Trey Parker, who flunked out of his degree, is the creative genius with the straw-bale hair; Matt Stone is the corkscrew-curled one in specs who knows how to run the ship. They hit it off, according Parker, because "we were the only ones who didn't want to make black-and-white films about lesbians". When they met, Parker was making a film called Giant Beaver of Southern Sri Lanka. For $125,000 he then made Cannibal: The Musical, on the back of which they moved to LA in 1995.

David Zucker, one of the Zucker Brothers who made the Airplane! and Naked Gun series, invited Parker to direct a spoof industry film, starring Demi Moore and Messrs Spielberg and Stallone, which was screened at a conference. Then a Fox executive saw an animated short of theirs, in which a snowman terrorises some Colorado kids, and commissioned a short video. He distributed The Spirit of Christmas, in which Santa and Jesus engage in a bout of kung fu, to industry friends as a seasonal greeting. Comedy Central won the bidding to make the series of the Christmas card. According to Parker, they chose the channel because "when we asked, `How do you feel about talking poo?' they said, `Love it!'"

On the back of the first series, they have sold more than $30m worth of merchandise. Its ads sell for six times the network's standard prime- time rate. Fans can download it on the Internet, where the stop-go animation doesn't lose much in translation, and where one website had millions of hits.

What is the secret of its success? The writing is witty and succinct, the plotlines satisfy and the characters develop but there's more to it than that. According to Frank Rich of the New York Times, the kids in South Park are "post-ideological", which may be just a polite term for a political incorrectness that America has embraced in gleeful relief.

"There's this whole thing out there about how kids are so innocent and pure," says Parker. "That's bullshit, man. Kids are malicious. They totally jump on any bandwagon and rip off the weak guy at any chance. They say whatever bad word they can think of. They are total bastards, but for some reason everyone has kids and forgets about what they were like when they were kids."

For several years now, most American sitcoms have been vanity packages for stars to play wittier versions of themselves: in Roseanne, Seinfeld, Cybill and Ellen, they even got to keep their own names. Cartoon characters don't have egos. Seinfeld may be the world's most popular show, but both King of the Hill and South Park have thumbed their noses at it. Boomhauer, Hank Hill's drinking buddy who spouts laval streams of southern semi-consciousness, calls it "the show `bout nuthin'".

Still, South Park's creators got a call from Jerry Seinfeld touting his vocal services. Chef, South Park's priapic token black, is voiced by Isaac Hayes, and Sparky, Stan's gay dog, by George Clooney. Jay Leno has done a cameo. Tiger Woods says he wants to. They offered Seinfeld the part of Turkey No 2 in the Christmas special.

Partly because they worry that they've not seen their fair share of the merchandising profits, Parker and Stone have moved into cinema. They have been engaged to write a prequel to Dumb and Dumber, and Parker has directed Orgazmo, a comedy set in the world of porn, in which he plays a Mormon stud and Stone a porn stagehand and photographer.

The worry for fans is that with all these distractions South Park will sell out. The second series started in the United States last month, although its makers went into it aware of the lure of easy formulae. "We would view success," says Stone, "as finally getting to the point where we get cancelled because no one gets it."

`South Park' begins on Channel 4 on 10 July