Successful trends (hip-hop, reality television, expensive bottled water) can last forever. But failures die a quick death and, like deposed Soviet leaders, get airbrushed out of history. Just last summer, CBS touted its next surefire hit, “Kid Nation,” a “social experiment” about a group of self-governing tykes on a ranch. They were so sure of its success that they moved it from the summer schedule to September in hopes that it would kick off the fall season with a bang.

It didn’t. And while “Kid Nation” didn’t fail ignominiously or as suddenly as “Viva Laughlin,” it merely limped along and vanished into memory’s leach pit without so much as a gurgle.

This brings us to “The Baby Borrowers” (8 p.m., NBC), another “social experiment.” This one invites five teen couples to test their relationships and their individual maturity by pretending to be adults. They have to set up a house and even fake pregnancy (by wearing something called an “empathy belly”) and attending prenatal classes. At the end of their lessons, real parents “volunteer” their babies and toddlers into the arms of the newly trained teens.

Once again, reality TV turns the natural order on its head. Television was supposed to be something to entertain the baby sitter. Now we’re supposed to be entertained by watching baby-sitting.

I guess we should be grateful that we see teens wrestling with life’s many big questions. But a bigger question remains: Is this entertainment? And why is the Tammy Wynette song “I Don’t Wanna Play House” running through my head?

It’s interesting to note that “Borrowers” had the exact opposite programming trajectory as “Kid Nation.” It was originally supposed to air during the regular season, but NBC yanked it and has dumped it into the summer, where it takes its place among such meaningful fare as “Celebrity Circus” (9 p.m., NBC).

¢ Host Neil deGrasse Tyson returns for six new episodes of “NOVA scienceNOW” (8 p.m., PBS, check local listings). The series discusses complex and challenging ideas and discoveries using a mix of interviews, graphics, humor and music in an effort to make science and scientists seem a little hipper and more understandable.

Tonight’s three stories include a discussion of the notion of “dark matter”; experiments that may help us rediscover lost memories; and new technologies that may enable us to detect fake photographs in the Photoshop era.

¢ The documentary “When I Knew” (6:30 p.m., Cinemax) interviews dozens of men and women from all over America about the moment they discovered that they were gay. The stories range from the amusing (a youthful fixation with the man on the Doan’s Pills box) to the tragic (parental disapproval and being disowned) to the poignant (parents who knew all along).

Tonight’s other highlights

¢ On three “Ghost Hunters” (Sci Fi), serious overtime (7 p.m.), lifeless in Seattle (8 p.m.), a really holy ghost (9 p.m.).

¢ Friends and colleagues gather for “The AFI Life Achievement Award: A Tribute to Warren Beatty” (8 p.m., USA).

¢ Scheduled on “Primetime: Crime” (9 p.m., ABC), the story of an “accidental” death does not hold up.

¢ Hairstylists compete on the second season of “Shear Genius” (9 p.m., Bravo).