Want to know the secret of The Simpsons' longevity?
It's because it remains timeless and relevant. That, according to executive producer Al Jean, is why The Simpsons is celebrating its 25th year, and why it could easily be around for a 50th season.
Jean has worked on more than 500 episodes of The Simpsons since it launched in 1989 - and that's despite the time he spent working on another animation project and also for Disney.
His time on The Simpsons includes the feature movie in 2007 and the theme-park ride at Universal Studios in Florida and Hollywood Studios.
Jean has ruled out a second feature film about Springfield's first family in the near future. But he envisages a 30th anniversary for the TV series, and possibly more celebrations beyond that.
"In show business you always treat every day as your last, but we're guaranteed through 26 (seasons)," he says.
"The deals are usually in instalments of four and the ratings are good, so I can't see why we wouldn't go to 30 ... and why can't we go to 40 or even 50.
"If making a second movie right now was a scale from A to Z, we are between A and B.
"The problem is there is so much work and there is no reason to do it unless it's good."
Little has changed on or off the screen on The Simpsons (which screens on TV3). The original voice actors remain, and the creative talent, including Jean, relentlessly pump out 22 episodes a year.
Jeans says that, as a "sitcom", the agelessness of animated characters makes it easy for fans to drift in and out of the series, and for new fans to tune in to the TV family their parents have enjoyed for more than two decades.
"One reason we have been on for 25 years is that if Bart was 30 years old and living with Homer it would be pathetic," he says.
"You basically have this template where people turn on the show and they're seeing the same thing they did five years ago and you're exploring new issues."
Jean pays tribute to the way the voices behind Bart (Nancy Cartwright), Homer (Dan Castellaneta), Lisa (Yeardley Smith) and Marge (Julie Kavner) have stayed the distance.
"Part of the reason for the success of the show, no doubt, is because of them."
Jean says many great sitcoms end when one of the stars wants to branch out and do something different.
He points to Cheers as an example: "I think that ended after a very long run because Ted Danson said they'd done enough."
But because The Simpsons is an animation, its voice actors have been able to forge lives away from the studio's recording booths.
"They have independent TV and film careers outside of The Simpsons and they don't feel they have to leave to do other things," Jean says.
The Simpsons has paid respect to some of the voice actors who have died by retiring their characters.
Phil Hartman's characters Troy McClure and Lionel Hutz have been retired since the actor's death in 1998, and Edna Krabappel has not re-emerged since Marcia Wallace died last year.
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