NEW YORK — Kids who live in the city learn their street smarts early. Not just "Don't talk to strangers," but also "Keep your hand on your wallet."

So when my 11-year-old wanted to ride his bike to the playground, I handed him a bike lock. My parting words were: "Use the lock. If that bike gets stolen, I'm not buying a new one."

An hour later, the bike was gone, swiped by a bigger kid who said, "Hey, can I see your bike for a minute?" and rode away.

"You fell for that?" I said to my son. "You'll be riding your scooter from now on."

But he wasn't upset. He was sure we'd get the bike back.

Incredibly, we did. We tracked the perp down using e-mail and Facebook, turning our big-city neighborhood into a nosy small town with a virtual front porch. The characters in this urban cybertale include a beat cop; the thief's relatives; and a chorus of online friends urging me on with girl-power posts like "Way to go Nancy Drew!"

Oh, and let's not forget my world-weary older son, who at 16 has already been mugged twice.

"You let him ride his bike to the playground?" he asked. "Don't you know it's too dangerous?"

But what's the point of having a bike if you can't ride it? I'm raising my children in a city, but I refuse to give in to urban realities, which unfortunately include crime. My kids will walk to school alone. They will take the subway. And darn it, they will ride their bikes!

A confession: I'd bought this bicycle to replace one that vanished from the hallway of our apartment building. This is not "Leave it to Beaver"-ville, where the Beave gets his stolen bike back. This is Brooklyn, N.Y., population 2.5 million, where my husband won't let me tie our dog up outside a store for fear she'll be dognapped.

Now, with two bikes stolen, I decided to file an insurance claim, but I needed a police report. There's usually a police officer stationed at the corner by the park, so I found him and asked how to report the theft.

I also mentioned that some other kids in the park had told my son that the thief just graduated from the middle school across the street. We knew his first name, too.

"If you file a police report, we'll have to arrest him," said the officer, making it clear he thought this was a bad idea.

"You should arrest him!" I said. "Let a judge decide what to do with him."

"Just wait a few days," the officer said. "You'll get the bike back." He said he'd already asked other cops on patrol to keep an eye out for the bike, a distinctive green and white BMX.

"That bike is gone!" I said. "The thief probably sold it already."

"The kid is local," the officer said. "He's around."

It was Friday. I agreed to wait until Monday to file the report.

Then I pointed to the middle school the thief attended, where my son will be a student come September. I asked if there was a lot of crime nearby.

"Try Catholic school," the officer said. "Much safer."

"Unfortunately not an option," I said. "We're Jewish."

Then it hit me: If the thief just graduated, he'd be in the yearbook!

I ran home, e-mailed some neighbors and put a note on my Facebook page asking if anyone had a middle school yearbook. On Saturday, a friend of a friend came through. My son found the perp in the yearbook, and then he found him on Facebook.

Like many teenagers, the culprit used no privacy settings. On his Facebook friend list, I saw someone with his last name, clearly an older relative.

I messaged both her and the kid: "You stole my son's bike!"

The relative messaged me back, asking how I knew I had the right person.

I sent her the name of the middle school the boy attended, and said I had his picture from the yearbook (true); that I knew where he lived (a lie), and that I knew where he was going to high school (a detail I cribbed from his Facebook page). I also promised to have him arrested if the bike wasn't returned by Monday.

"I'm so sorry," she responded. She said she'd let the boy's sister know.

On Sunday the thief started messaging me. First, he wanted to be my Facebook friend. Then he said he took the bike because another kid told him to. Then he gave the street where the bike was locked up, along with the combination for the lock. Finally, he wrote: "Where do u live at?"

Well, I wasn't going to friend him, or tell him where I live. But the bike was where he said, and the lock code worked.

"I knew we'd find it," my son said, as if it was no big deal.

On my Facebook page, I wrote: "Beth found her son's bicycle thief on Facebook. And now she has the bike back. :)"

My Facebook friends chimed in: "Well done super mom!" "The best Facebook story ever." "Impressive feat of FB detective work!" "You're the fourth Angel!" (As in Charlie's.)

I messaged the boy's relative, thanked her for intervening, and told her the boy was lucky to have a caring person like her in his life.

I had to hand it to the cop. He was right: The kid was local, and we found the bike.