Many parents scratch their heads and wonder why their teenagers have to have a particular pair of jeans or a pair of Ugg boots or a trucker cap -- right now.
Who decides what's cool in the world of teenagers? And what makes a trend suddenly, like, so over?
Malcolm Gladwell, a journalist and best-selling author of the book "The Tipping Point," studies this very phenomenon -- the rise and fall of trends.
Gladwell said one trend that has been very clearly traced is "valley speak."
"It's where they engage in what's called 'up talk' -- you know, how teens talk and they end every sentence with a question?" Gladwell said.
Gladwell said up talk appears to have started among Australian surfers in the 1960s, who eventually came to California.
"And all kinds of little teeny boppers would cluster around them and try to emulate them," Gladwell said. "Eventually, it finds its way among almost every high school and mall in the country."
Trends are a virus, Gladwell says, and somewhere out there is the "patient zero" of low-rise jeans. The trick for marketers is finding her.
In the '90s, a cottage industry of "coolhunters" sprung forth, claiming to be the fortune tellers of teen taste. Some were right, most were wrong. So these days, most fashion houses go right to the source.
Sophie May is a ninth-grader who loves to shop. But unlike most ninth-graders, she rides a corporate stretch Hummer to the mall, where designers and consultants study her every move.
"Every time someone copies me, I'm flattered, but I change it," Sophie said.
Sophie is one in an army of Teen Vogue "It Girls."
"They are influencers, and the fact is that when you reach one, you reach so many other teen girls," said Gina Sanders, an editor at Teen Vogue magazine.
If the new Betsy Johnson cell phone is going to sweep the nation, for example, it will have to impress them first.
"I can walk into school and say, 'Look what I've got,' and no one could say, 'Oh yeah, me too,' " said one "It Girl."
At Los Angeles' ultra-hip Kitson boutique, they have a much more efficient way of reaching teen girls -- get just one hot celebrity to be seen in something.
Take the Kurtz Army hat. Kitson manager Sara Giller said the olive military-style caps weren't moving, until one crucial moment.
"Lindsay Lohan wore it," Giller said. "Those Kurtz hats weren't doing very well at all, and Lindsay Lohan came in with her friend Paris Hilton, put one on and was seen walking out -- and it went like crazy, like hot cakes."
So did she wear it because it was cool? Or is it cool because she wore it? Either way, it is only a matter of time before this trend sweeps the nation, becomes uncool and hits the discount bins -- or in Gladwell's term, hits the "tipping point."
How can you tell when a trend is over?
"When it's on your grandma," said Jane Buckingham, the president of Youth Intelligence, a consumer research group. "Nothing against your grandma, but when you're grandma's wearing it, it's time to move on."
But don't be so sure, Jane. Remember the "It Girls?"
Uber-teen Sophie May said half the stuff in her closet is from her grandmother.
"I'm wearing my grandmother's shoes right now, and I have bags of jewelry from my grandmother's house," she said. "It's at-home vintage shopping."