And yet "Mean Girls," even as it keeps us laughing, treads into some painfully realistic territory, perhaps because the teenage turf it's treading hasn't changed all that much in the past 50 years. There have always been, and probably always will be, social hierarchies in high school. "Mean Girls" reminds us, mischievously, how little those anthill social divisions mean in the grand scheme of adult life, even as it brings back how much "fitting in" matters when you're a teenager.

Lindsay Lohan is Cady, who has been home-schooled for years not because her parents are religious freaks but because they're zoologists (they're played, with generous dollops of good-liberal-parent earnestness, by Anna Gasteyer and Neil Flynn). For the first 15 years of her life, Cady has lived with her parents in Africa, which means that in her case, the notion of high school as intimidating foreign territory isn't just a metaphor -- it's piercingly literal. Friendly, open and trusting, Cady is lost in a place where neither the administrative rules nor the unwritten social ones make any sense. She doesn't realize, for example, that she needs a hall pass to use the bathroom; she has no idea where to sit in the cafeteria at mealtime, which means she eats her lunch crouched in a lavatory stall, alone.

But two fellow misfits, the gentle, lumbering, almost-openly-gay Damien (Daniel Franzese) and tough-but-sensitive Janis Ian (played by Lizzy Caplan -- her character's name is the picture's most obvious acknowledgment that grown-ups are actually the ideal audience for teen movies), quickly take Cady under their wing. Their first act of kindness is to present her with a map of the lunchroom, so she'll know which tables have been staked out by which cliques, among them the Preps, the JV Jocks, the Asian Nerds, the Cool Asians and the Sexually Active Band Geeks (the last of whom are shown pawing, groping and sucking at one another with oblivious abandon).

But the most elite table is the one presided over by the school's royalty, also known as the Plastics: Gretchen (Lacey Chabert) is the one who gleans the juiciest gossip about her classmates and wields it like a pink-fur-covered club; Karen (Amanda Seyfried) is the ditzy one who looks adorable and follows the group's rules unquestioningly; and Regina (Rachel McAdams) is the group's (and the school's) Queen Bee, the blond bitch-goddess whom everyone looks up to, emulates and fears.