In 440 BC, the Greek historian Herodotus first described a trick that spies used to send hidden messages. They'd write something on the wooden back of a wax tablet, then cover the message with wax bearing its own message. If enemies intercepted the tablet, they wouldn't suspect it contained anything strange. It's called steganography: hiding one message inside another.

Two thousand years later, teenagers are doing something similar to communicate with one another—on Facebook.

What turned teens into Greek spies? The parent problem. If you're in high school these days, a lot of your socializing happens online, but your parents usually insist on being "friended" so they can check what you're posting. This creates a communication dilemma. You want to post candid updates about your life so your friends know what's going on—but not so candid that your folks catch wind of it.

The solution is what researcher Danah Boyd has dubbed social steganography. Teenagers now post status updates that have two layers: A bland surface meaning intended for parents, and a deeper, richer significance that can be decoded only by close friends.

For example, Boyd interviewed one girl who was going through a breakup while on a class trip and wanted her friends to know but not her mother (who'd "have a heart attack"). So the teenager posted the chorus of a black-humor Monty Python song sung by a group of men who've been crucified. ("Always look on the bright side of life / Always look on the bright side of life!") Her close friends, being fans of the movie, understood the reference and immediately messaged her to offer support. But her mother didn't know the film, so she thought the lyrics were genuinely cheery and posted a response saying she was glad her daughter was happy.

Posting lyrics to communicate your mood is one of the most common social steganographic tricks, because teens are fluent in pop culture in a way their parents aren't. What teenagers are doing reminds me of Washington's "dog whistle" politics, in which politicians deliver speeches that sound bland but are laden with meaning aimed at their base. For instance, Republican kingmaker Lee Atwater used to advise candidates to use phrases like "states' rights" and "forced busing" to incite racial fears among white voters without actually using offensive language.

Obviously, one could regard the emergence of youth steganography as yet more depressing evidence of how dangerously overcomplex the web has made teens' lives. But frankly, I'm kind of awed by the rhetorical sophistication of today's teens. They are basically required to live in public (you try maintaining friendships without an online presence), but they crave some privacy, too. So they've taught themselves to hack language. They hack systems, as well: Boyd has also found teenagers who "deactivate" their Facebook account when they log off so nobody can see their stuff or post comments. Then they "reactivate" it when they want to go back online and interact with friends. Presto: They create a virtual club where they control the operating hours. Color me impressed.

Social steganography also illustrates that, for all the raging popularity of networking online, people still aren't happy with their tools. We need better options, more experiments, greater innovation. Consider the design of the Latin American network Sonico, which lets users organize their contacts into three simple buckets—workmates, family, and friends. Postings intended for one group aren't shown to another.

That sure beats sending around wax tablets.

Email clive@clivethompson.net.