When asked how long she plans to stay at Saturday Night Live, Tina Fey — the sketchcom's first female head writer — is, well, sketchy. "Thirty more years?" she jokes to TV Guide Online. "I don't really know." Fortunately, the gig leaves Fey time for side projects, like her new big-screen comedy, Mean Girls (opening Friday).

She wrote and co-stars in the Heathers-esque film, which draws from her own rotten high-school experiences. "I had girls threaten to kick my ass," Fey admits. "I had a lot of friends and we were all sort of AP students. We were our own sad little clique. We thought we were supercool, but we weren't.

"Then, there were the girls who were sort of famous," she continues. "They were really popular and everyone knew everything about them. And [I was] like, 'That one is not even cute. How did she get that job? Just because she has nice Benetton clothes?'"

Fey says the witchy 'tudes in Mean Girls were also inspired by Rosalind Wiseman's nonfiction book, Queen Bees & Wannabes. "When I read [Wiseman's New York Times article on which the book was based], I immediately thought, 'This is something that interests me and this is something that I could write about.' Because it was about girls," she enthuses, "and it was really nasty and violent and that appealed to me."

The movie's most-popular girl, Regina, did get one character trait from an unlikely source. "One of Regina's big moves in the movie is actually based on my mom," Fey laughs. "My mom has this habit that if she sees a lady in a really ugly hat or a glittery sweatshirt, she will go out of her way to be like, 'I love your sweatshirt.'

"I'm like, 'Mom, that's really mean.' She always says, 'Why? Clearly she wanted someone to acknowledge her. She picked it out with a huge teddy bear on it'." Trust us, Tina, Mother probably knows best.