Doh! The Simpsons still young and yellow at 20

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America's most famous dysfunctional cartoon family, The Simpsons, this week marks two decades of making the world laugh while offering alternative television therapy to millions of fans.

The now distinctive yellow characters of Homer, Maggie and their children, the intellectually-challenged Bart, his smart sister Lisa and pacifier-sucking baby Maggie first burst onto American TV screens on December 17, 1989.

Over the past 20 years, they have entered into the national and global consciousness as an icon of television entertainment.

The Fox network show's influence on popular American culture was highlighted when The Simpsons were idolized on US postage stamps earlier this year, and in November Marge Simpson was the cover girl for Playboy magazine.

The gravelly-voiced Marge, whose upswept blue hair-do defies gravity, became the first cartoon character to grace the cover of the magazine, more known for featuring movie stars, athletes and other celebrities in states of undress.

The show's success has surprised even creator Matt Groening and executive producer Al Jean, the creative pens behind the family which lives in the shadow of a nuclear reactor in a fictional town called Springfield.

"I knew that the show would be a success, but I didn't know that it would be so big, last so long and become a global phenomenon," Groening said earlier this year.

"The Simpsons immediately struck a chord with viewers across the country as it poked fun at itself and everything in its wake," Fox studios said in a statement.

"With its subversive humor and delightful wit, the series has made an indelible imprint on American pop culture, and the family members have become television icons."

The series is now the longest running comedy in US television history, and has received numerous awards.

Fox is planning to mark the 20th birthday with a special documentary about how the world views The Simpsons to be directed by Oscar-nominated filmmaker Morgan Spurlock, star of "Super Size Me" and "30 Days".

"When they first called me about this, I thought it was a prank and I hung up," Spurlock said.

"And then my agent called back and said, 'No, no, this is for real,' at which point I fainted. Then when I woke up, I called everyone I knew because it was the coolest thing I could ever get to do in my career."

The documentary "The Simpsons 20th Anniversary Special in 3D on Ice" will air on January 10.

Numerous celebrities have been lampooned on the show over the decades, including most recently French President Nicolas Sarkozy and his glamorous wife, former model Carla Bruni.

They appeared in a November episode entitled "The Devil Wears Nada" in which bungling paterfamilias, the doughnut-loving Homer and his colleague Carl Carlson visit Paris and bump into Bruni, a cigarette-smoking femme fatale in a stylish ballgown.

After a brief exchange of pleasantries the Bruni character throws herself into Carl's arms and declares: "I want to make love, right now."

While Britain's former prime minister Tony Blair and Fox owner Rupert Murdoch recorded their own voices for their appearances and escaped with a gentle ribbing, the harsher Sarkozy parody appeared without their consent.

So far though no US president, current or former, has ever accepted an invite to appear, although President Barack Obama did send a nice rejection, Jean said earlier this year.

Premiered on the then fledgling Fox channel as a half-hour Christmas special on December 17, 1989 and then as a regular series from January 14, 1990, the success of the Simpsons is a TV legend. Today it is broadcast in 45 languages.

And in 2007 they hit the big screen for the first time with a feature-length film "The Simpsons Movie."

The epic took four years to complete and Groening and Jean said a new film was only a distant possibility.

"I suppose someday there might be a sequel, but not yet," Groening said.