I REMEMBER when the big thing was for kids to pass love notes.

Tres 20th century.

Now it's all about sextexting - using the latest technology to send a nude or semi-nude photo to a crush.

I have my teenaged stepson to thank for clueing me in on this new-fangled, teenage courtship ritual. He'd been sitting, all slumped on the couch, looking remote and noncommunicative. I asked him what was going on at school. Instead of getting the usual "nothing" response I expected, my ears were filled with a crazy tale about a nude picture on a cell phone that got passed all around his high school and some kid's getting suspended.

That was my introduction. Since then, I've learned that nearly a fifth of teens have sent or posted online nude or partially nude photos or videos of themselves, according to a new study by the National Campaign to Prevent Teen and Unwanted Pregnancy and CosmoGirl.com.

If you consider adults ages 20 to 26, the number goes even higher. An estimated 36 percent of young adult women and 31 percent of young adult men have engaged in sextexting.

It's easy to see why passing risque photos around is popular. What high-school kid doesn't have a cell phone these days? The availability of all that technology, plus raging hormones, plus immaturity, plus the prevalence of hook-up culture, makes for a blowup in the making.

Morality aside, the big issue comes down to control. Kids need to be reminded that once an X-rated photo of themselves gets out, it can turn up anywhere. A third of boys surveyed admitted having seen an image that was meant to be private.

Earlier this year, there was that case in Seattle that made national news in which two high- school cheerleaders were suspended after nude photos of them were texted all over high school. And there was the incident with "High School Musical" star Vanessa Hudgens, who sent a nude photo to her boyfriend that wound up on the Internet.

All I can say is, I'm glad all we had when I was a teen were Polaroid cameras. In fact, I'm not sure we even had those . . . talk about the dark ages.

Sexy Singles news

Speaking of sexy images, the winner of this summer's Daily News Sexy Single contest has come up with a novel way to use hers to give back.

Speaking of sexy images, the winner of this summer's Sexy Single contest has come up with a novel way to use hers to give back.

Gail Kasper, who was among the unattached folks featured in this annual singles feature, is selling posters of herself (below) to benefit charities such as Pawtographs, an animal-welfare group she founded.

"I wanted to turn something into a positive. That's what led to this," said Kasper, who has done poster signings at Philadelphia Veterans Administration Medical Center and McGuire Air Force Base. "These three posters are designed to create awareness, deliver a message and also make a difference.

"I'm trying to heal the world from, I guess, all angles."

Kasper, a motivational speaker and TV host, is hoping to send 5,000 posters to troops in Iraq. (By the way, she could use a sponsor to help her pay for cardboard cylinders to mail them in.) Her posters are available through www.thebeautifulposters. com. *