1. Beware of girls with big hair (they hold the most secrets).
At least according to kohl-eyed Janis Ian. Dolly Parton (and we here at Birchbox) have another theory.
2. Never fake dumb just to get a cute guy's attention.
Just like Cady Heron, who asks her crush Aaron Samuels to help with her math homework, you'll end up with F's.
3. Never trust a Swedish nutrition bar.
Think twice before accepting any foreign so-called weight loss food, especially if the list of ingredients is harder to pronounce than an Ikea catalog.
4. You can pull anything off as long as you do it with confidence.
After having holes strategically cut into her tank top, Regina struts through the girls' locker room with enough swagger to spark a wave of copycat fashionistas. Lesson learned: Own what you wear.
5. When in doubt, just order cheese fries.
Even if you're on a mission to "lose three pounds," real friends would never throw shade on your craving for disco fries.
6. When cleaning up after a house party, remember to take the vases back out from under the sink.
After making this rookie mistake following a weekend rager, Cady ends up on house arrest. Thankfully we saw this movie in time to learn from her misfortune.
7. Always look both ways before crossing the street.
After Regina's untimely accident—so close to prom!—we'll never cross the street again without making sure it's clear of speeding school buses.
8. Some people will always be known by their first and last names.
Regina George. Gretchen Weiners. Janis Ian. Notice a trend? From the popular "plastics" to the unknown outcasts, you will always remember your high school classmates' names just as they appeared in your yearbook.
9. Hair always looks sexy pushed back.
But seriously, don't you think that Aaron's hair looks sexy pushed back?
10. Fetch is most definitely not going to happen.
Ten years later, that's still the case.
Photos: Paramount Pictures; All GIFs: meangirlsgifs.tumblr.com