Nancy Cartwright is the boy who never grew up. The 52-year-old voice actress is preserved on screen as cartoon character Bart Simpson, who celebrates his family's 20th anniversary this year yet remains 10 years old in his short blue trousers.
"I'm five times Bart's age and I'm still 10, it's crazy," laughs Cartwright. While Bart is spiky-haired and sharp-tongued, Cartwright is warm and glamorous -- but she credits Bart with keeping her young.
"It helps me keep youthful because my biggest fans are these little guys and if they see what I look like and I turn around and say, 'Hi man, I'm Bart,' in the voice, they're shocked. It's awesome, I love it," she says.
Cartwright can't believe that what started out as short 'bumpers' on The Tracey Ullman Show in the US is now a global phenomenon which boasts dozens of characters living in fictional town Springfield: "Back then the family was not fleshed out. Bart was the antagonist of the group, Lisa was the sensibility, and the mom was doing the best she could with the dad mostly beating up on Bart, but when we went to the half hour we started getting the taste of the possibility that this could expand.
"At the time I wasn't looking far into the future. I was the lead because Bart was definitely positioned as the star of the show, which I thought was quite cool, but there was no concept of 10 years, 20 years -- it was just 'I hope we get picked up for another 13 after this'," she says.
Not only has Bart's character been fleshed out, but Cartwright thinks he seems more of a cheeky tyke than a real villain as times change.
"Twenty years ago Bart was perceived as worse than he is today and I'm glad Fox has kept a little more control on his behaviour, more than some of the other networks have done with their cartoon characters, even though we now get away with things we couldn't have done back then," she says.
It was Bart's devil-may-care attitude which drew Cartwright -- who honed her voice talents by doing public speaking while at school -- to the role.
She describes her audition.
"They were looking for an eight-year-old child, who was a girl, the famous Lisa Simpson, but I saw the description of Lisa there and her little monologue and next to that was a picture of Bart, and for Bart it said: 'Ten-year-old school-hating underachiever and proud of it'. Now, between eight-year-old child and what I just said, what would you rather do? So I read for the part and that was it, they hired me on the spot."
Playing a boy was not a big deal for Cartwright, who said she was often mistaken for a boy when growing up.
She explains: "I was about seven when people started making comments about my voice, and being seven and having short hair I had an androgynous quality, so people didn't know whether I was a boy or a girl. But I just used my voice to make people laugh, and because it made people laugh, it made me happy."
That casting has led to a 20-year working relationship with the show's creator Matt Groening, its executive producers -- among them Al Jean and James L Brooks -- and of course, show stalwarts Dan Castellaneta (who voices Homer), Julie Kavner (Marge) and Yeardley Smith (Lisa).
The Simpsons is now broadcast all over the world and currently stands as the longest-running comedy in television history.
Did Cartwright ever imagine she'd be working on something with such universal appeal?
"It wasn't part of my thinking," she declares. "It wasn't part of our culture. I grew up with The Flintstones . . . so the only cartoons for adults I knew about were the racy ones."
The 21st season of The Simpsons launches on Sky1 on Thursday, January 14