One of my twins came home from school in a badder-than-average mood last week.

Even when everyone is in a good mood, doors slam with such regularity in our house that it sounds as if poltergeists have the run of the place. Still, there was something different about the G-force applied to the door that evening.

For once, the slamming door didn't sound like it was directed at me, his mom, his older brother or his twin. The house-shaking slam didn't result from the usual teenage melodrama, either.

I listened to his entrance from my second floor office. There was no happy reunion with the dog who greeted him with friendly barks at the door -- just heavy-footed stomps through adjoining rooms on the way to the kitchen.

Do your chores, I shouted. It's my way of saying hello to my boys. He didn't respond with the usual whine. When he stomped through my office on his way to the attic, I asked what his problem was.

My son looked at me with eyes brimming with rage and a few tears and said, "Why do people have to steal so much?" He punched the wall in frustration. It was a cri de coeur from a boy who doesn't wax existential about many things.

After some prompting, he filled in the blanks with an anguished tale of woe. An hour earlier, another teenager from the Hill District he'd often exchanged friendly banter with had snatched his brand new $149 iPod nano near the Wood Street T-station, Downtown.

Startled from his dreamy complacency after a long day at CAPA, my son chased the thief and his three accomplices halfway up Strawberry Way, the alley-like street that juts off Liberty Avenue, before thinking better of it. Four-on-one makes bad odds even for a well-trained martial artist like him.

My son described the thief as taller, slightly older, and upwards to 250 pounds. He was also wearing a hoodie. Gee, you'd think he'd have been more suspicious when someone fitting that description asked to "hear" what he was listening to.

"Is this why I've been paying for karate lessons all these years?" I said, giving into an immature compulsion to needle him. Instead of laughing, he punched the wall and retreated to the attic.

After apologizing, I told him that I was relieved that he wasn't the kind of macho idiot who'd fight four people in an alley over a status symbol. A molded piece of plastic isn't worth dying over.

You're alive, I blurted out, suddenly relieved after imagining how the scenario could have played itself out. Christopher Norman is still with us. Last year, 15-year-old Christopher Rose was murdered by a gang in Brooklyn during a similar iPod robbery.

This year, iPod robberies accounted for a 10 percent increase in robberies in New York and a 34 percent increase in Los Angeles. When it comes to teen-on-teen crime, iPod thefts lead the pack because they are crimes of opportunity.

IPod users are usually unaware of being stalked because one of their major senses is heavily occupied. The boys who stole my son's iPod nano were simply following a large and unoriginal crowd.

"Why did he steal from me? I've never done anything to him," Chris asked, as if his purity of heart means anything in the real world.

I explained to him that to a criminal who subscribes to the politics of envy, he looks relatively "privileged." Chris dresses well, has perfect teeth, carries a cell phone, and can speak clearly without resorting to urban patois and slang to get his ideas across.

His full backpack marks him as someone who (probably) takes school seriously. His well-coiffed afro, though half the size it was last year, is straight out of the 1970s. He's a handsome, exotic-looking boy who doesn't look as if he's experienced much of this city's mean streets. If I weren't his father, I'd probably mug him, too.

Once again, I couldn't resist teasing him and reminding him of a recent argument we had.

"Your stolen iPod is full of songs with uplifting titles like 'Every Day I'm Hustling.' What do you think those boys were doing when they took your iPod? Now they have something to listen to when they're hustlin' and robbing."

He smiled wryly and ignored me. I'd suddenly lost whatever moral authority I had. The next day, I took Chris and his twin brother to see "Jet Li's Fearless," a martial arts movie about the unintended consequences of revenge.

Both boys picked up on the film's schizophrenic message about peace delivered via healthy dollops of excruciating personal warfare. They agreed that Jet Li's character should have exercised more "restraint," but wondered what kind of audience a nonviolent karate movie would attract.

As soon as they got out of the car, they started fighting on the front lawn. Is this why I've been paying for karate lessons all these years? In a word, yes.