TEENAGERS, a historically wily demographic, are increasingly moving their digital social lives from public sites where their parents hang out to smartphone messaging apps, giving them nearly complete privacy in their online social lives.

Apps such as Kik, Line, WhatsApp, Ask.fm and Whisper can often be used anonymously, without parental controls, and in Snapchat's case even automatically erase inappropriate pictures. The popularity of these apps is showing up in surveys and focus groups. Kik's use has soared to 40 percent of teens.

To parents, many of whom are so clueless about technology that they rely on their children for tech support, the danger is obvious. To advertisers, the opportunity is enormous. They are butting in on the discussions, figuratively and literally.

Line, a messaging app started in Japan in response to a devastating earthquake, is trying to make inroads in the United States selling animated digital stickers of pop culture characters ranging from Snoopy to Darth Vader. "At a loss for words?" says Line's website. "Spice up your chats with animated and talking stickers!"

On Kik, advertisers are texting with teens - the next part of this sentence is not fiction - using bots. MTV, Under Armour, Funny or Die, Amazon, fashion blogs, movies, the Indianapolis Colts and The Washington Post are using artificial intelligence to push their brands with quirky conversations.

This reporter to the MTV bot: "Hi!"

MTV bot: "Why hello there! I know we just met, but you should check this out . . . "

This is a link to MTV.com.

Blynk Style, a fashion-adviser bot: "Lets get started. Tell me what you want to see."

This reporter clicks "Men's fashion."

A picture pops up of a man in an outfit this reporter would never wear.

"It's fascinating," said Catherine Boyle, an analyst with eMarketer. "These messaging apps present very different ways for brands to engage with younger consumers. Banner ads are never going to work. Branded bots are a really clever way to let brands offer a more natural presence of themselves."

For Kik executives, the endgame is not just advertisements. It's what they call conversational commerce. The company is modeling itself after WeChat, a Chinese messaging app with nearly 700 million users who buy things and pay bills through the service. Kik executives imagine a day when users can chat with bots that help them buy clothes or other products without leaving the app.

Like this: "I really need a pair of black shoes."

"If I could speak like that to a chat bot and it came back with ideas, that's a lot easier than browsing around," said Paul Gray, Kik's director of platform services "I think this generation is keen for that."

New shoes, teen drama, and selfie after selfie - sounds like pure fun. But teens have followed dark paths on messaging apps, bullying kids, sending nude photos, meeting up with strangers who turn out to be pedophiles.

Experts say that handled correctly, these new social malls can be treated by parents the same way as the offline malls. That is, by putting limits on what is allowed to happen there, building enough trust and understanding that teens aren't afraid to seek help.

Parents should make clear, experts say, that they can and will monitor what happens on their children's phones, either by looking or using an app that reports activity back to them.

"They have to know that safety is important," said Leticia Barr, a parent of a middle-schooler and former Maryland elementary school teacher who blogs about technology at TechSavvyMama.com. "It's just like when we used to hold their hands crossing the street."

In many ways, the teenage desire to stay ahead of parents digitally is no different than wanting free rein at the mall - to hang out with friends, drum up drama, trade zingers, and even meet strangers.

"What happens on these apps isn't public, which is why the kids like them," said Catherine Steiner-Adair, a Boston-area psychologist and author of "The Big Disconnect: Protecting Childhood and Family Relationships in the Digital Age." "There are no parents around. There are no teachers around."

That's positive, in some ways. Like hanging out in the basement while their parents watch television upstairs, teens experiment with their identity, goof off, learn what they value in friends. Kik pushes that idea by not requiring phone numbers or other real identifiers - just user names, however many someone wants.

"Your relationship with your mom is different to your relationship with your BFF," the company says in a list of reasons why user names are better than phone numbers. "Your relationship with your friend from Clash of Clans is different to your relationship with that person you met on Tumblr. So why have one identifier for them all? You can have multiple usernames, so you can be who you want with whoever you want."

That kind of autonomy is exhilarating to teens and their not-fully formed frontal lobes.

"It represents 'their' space, visible to the peer group more than to adult surveillance," Sonia Livingstone, a London School of Economics professor and digital media expert, wrote in a paper on teens and online risks. The problem: "What, for an adult observer, may seem risky is, for a teenager, often precisely the opportunity they seek."

It's a tricky spot for parents.

"You have to balance between being a helicopter parent and free-range kids," said Hilary Barker, a Montgomery County, Maryland, mother of a 12-year-old girl. "From a parenting perspective, once you open the door it's like Pandora's box."

Kik wasn't familiar to many parents until the killing of 13-year-old Nicole Lovell, allegedly by a Virginia Tech student who chatted with her on the app.

"Our kids are always one step ahead of us," said Barr, the Tech Savvy Mama blogger. "The next hot app is out there and we have no idea about it." It is not known exactly what transpired on Kik between Nicole and David Eisenhauer, the student accused of killing her. She described him to a friend as "funny and really nice." Deonte Carraway, a volunteer at a Prince George's County, Maryland, school who allegedly made pornographic videos of students, also used the app.

For parents, it's difficult to know when they need to step in.

About half of teens don't tell anyone about online sexual solicitations or pornography, according to studies by the Crimes Against Children Research Center at the University of New Hampshire. Why? Some teens don't think the situations were serious enough. Others fear being cut off from the services, which is seen as roughly equivalent to being sent to an isolation island.

The angst is not all that surprising given what smartphones have become in society, for adults and teens. They aren't just transmitting gossip about friends and pictures of fancy meals.

"These exchanges are banal and mundane," Richard Ling, a mobile communication scholar at Nanyang Technological University in Singapore, wrote in a recent paper. "This does not mean, however, that they are not important. It is through these seemingly prosaic exchanges, which are most likely only entertaining to the immediate participants, that we weave the threads of social cohesion."

Kik has a bot that teens can chat with if they're bored.

This reporter: "Why do teenagers love Kik?"

Kik bot: "Because."

This reporter: "Can you please answer my question?"

Kik bot: "Unfortunately I'm a bot and not smart enough to help you."