The kid known as Berg shows up, in their memory's eye, with his impish smile and his tuft of dirty blond hair atop an otherwise shaved scalp and that brilliant mind, which bounced about like a pinball. Even now his friends smile.

"I remember this freshman comes walking out to the first band practice with his sousaphone and all of his philosophies and funny voices and right there, you knew he was something else," says Luke Lorenz, who was a high school junior at the time. "We went away for two weeks of practice and that first night Berg takes some scrap of aluminum foil and a Walkman and constructs an alarm system for our cabin.

"If you triggered it, a taped voice started yelling: GET OUT OF HERE! GET OUT OF HERE!"

This was Bergology, aka the Science of Berg. Now Nick Berg is dead and all that's gone forever. He was beheaded by black-cloaked guerrillas in Iraq for the crime of being, what? An American? A Jew? An idealist naive enough to wander into an American occupation zone gone ominous? His friends, drawn back to their leafy home town west of Philadelphia by news of his death, can't make sense of such a life extinguished.

"He was funny, outgoing, dramatic, compassionate, intuitive, brilliant." Lorenz, who at first had not wanted to talk because he was exhausted and because it's so painful, cannot stop himself now. "You throw a whole bunch of those adjectives in a blender and you know what you come up with? Berg."

It's a strange business, to reconstruct a person's life from fragments of memory and photos and videotapes and deep sorrow. But Berg seems to come striding whole out of memory. He's the kid who built a practice transmission tower in his suburban back yard -- rising at least 35 feet high above the oaks and sycamores. The teenager who would ride his bike a hundred miles on a whim. Who cooked spaghetti dinners and drove his friends to the junior prom.

He was a good enough student to get into Cornell University and restless enough to drop out a year short of graduation. He was the young man who climbed cliffs and read deeply in history and mythology and, of late, in Judaism and Islam. He was the talk of his friends, who took a fierce pride in the course he charted.

What's up with Berg?

Did you hear from Berg?

Oh, man, let me tell you about Berg.

"Nick was an explorer," says Peter Lu, a slight, close-shaven high school chum now in his fourth year as a physics PhD student at Harvard. "He went where no one else did. If there was a path, you could bet Berg wouldn't be on it."

The Berg family's home in West Chester sits on a long and winding drive, the tree boughs heavy with leaves, the azaleas in full hallucinogenic blooms of red, pink and magenta. On the block, children's bikes are discarded on porches and basketball hoops sit at curbside and American flags float soft on spring breezes. You can identify the house by the camp of television trucks and sound-booms, and the piles of flowers by the door.

Taped to the mailbox is a photo of a stocky, buff kid in a cut-off T-shirt, running shorts and glasses. His smile is broad.

"Ah, man, this kid was going to be something else," says Bruce Hauser, a neighbor, who was the first African American to move into the neighborhood three decades ago. The Bergs welcomed him with dishes of zucchini and invitations to dinner. "He was friendly, he'd help anyone anytime, and then I'd see him climbing that practice communication tower he built in that back yard."

School came easy to Nick, whose parents were schoolteachers. By the time he got to high school, he had found a niche among the science kids, joining the National Science Olympiad and the debate team. Lu remembers Berg as being far more accomplished than him in physics. He also recalls that his friend would as soon read about Byzantium or Greek philosophy as study his equations.

"Nick understood physics better than any of us, but he didn't want to get concerned with the details of grades," Lu says. "The real reward was understanding it."

Charles Wood, who taught physics to Berg in high school and coached him in the Science Olympiad, recalls an inventiveness that was almost unnerving. "Nick loved to learn. But if he was around, you knew it. He kept you on your toes."

A home video shows Berg at a science fair in Georgia, hamming it up for the cameras, interviewing a friend, putting on a Ross Perot-like voice, cracking up himself and everyone around him. His buddy Jeffrey Becker, now a PhD student at the University of North Carolina at Chapel Hill, recalls sitting around a gym at some science competition. The students were bored, waiting on the judges, and Berg picked up his Walkman and slipped off. Soon enough, Led Zeppelin came ripping over the judges' public address system.

"I have no idea how he did it but that music was blasting," Becker says. "He brought that wonderment and joy to all of his science. Little kids loved him."

Berg set off for Cornell after graduating from high school. He passed a few years there, doing well enough in classes. But friends heard in his voice a spirit sagging. One winter night he called his friend Lorenz and said: I'm biking your way. It was 100 miles away. Lorenz drove and picked him up halfway. "I pulled over the car in northern Pennsylvania and I got out and said, 'Nick, look at the Milky Way.' "

The two friends stood there silently for half an hour. A few weeks later, Nick was off to Africa. He ended up in Uganda, a poor nation on a poor continent, taking soil samples, trying to develop a brick that would not require water. He wanted to build communications towers, to spread knowledge, so that all those kids he was befriending might have a chance at something better.

Nick traveled to Africa at least twice, returning each time with only the clothes he wore. He had given everything else away. He told stories of standing in a village market in northern Uganda, talking local politics in his impassioned way with a Muslim cleric. Are you a Christian, the cleric demanded. No, Nick said, I'm Jewish.

The cleric stared at him -- and returned to their discussion.

Berg returned and dropped out of Cornell, not far from graduation. He attended Drexel and the University of Oklahoma in the next year or two, never quite getting that degree. "I think he just wanted the access to books and labs that colleges gave him," Lorenz says. "Berg taught himself."

The Bergs, by all accounts, are secularized Jews. But one of Berg's friends, Aaron Spool, was an orthodox Jew, and a few years back Berg asked him for some readings from the Torah. A few weeks later, Berg called back with a set of theological questions that, in their complexity, took his friend aback. Berg carried a yarmulke with him to Iraq, friends say.

From a message left by Berg on Lorenz's answering machine a couple of years ago: "Luke, I just had this idea. I wanted to run it by you, but I don't have a lot of time. I've gotta go check out some Roman mythology."

Berg began to work atop communications towers in Oklahoma, and after returning to Pennsylvania he founded his own company, named after a figure from mythology: Prometheus Methods Tower Service Inc. He based the company, which helped maintain communications towers, in a Lancaster County farm owned by Jay Scott Hollinger, his foreman.

"Nick was just an incredibly kind and intelligent guy," Hollinger said. "He had no girlfriend, he didn't drink, he didn't watch television. He just worked and went rock-climbing and read everything."

Berg's parents and brother and sisters are liberals and anti-Bush and antiwar in equal measure, according to friends and the bumper stickers on their cars. Nick was not. He believed in President Bush and the liberation of Iraq. He went out to play football with Lorenz and another friend last December. It was a balmy day and, as always, they talked about everything. Toward the end, Berg spoke of going to Iraq, where he would climb and fix communications towers -- and put American flags on top. He wanted to make some money, and he wanted to be part of building something good.

"He believed that you can't understand anyone if you fear them," Lorenz says. "The power of his vision was much greater than our fear for his safety. He believed he would emerge safely."

There were two trips to Iraq and his e-mails from there read so often like travelogues of a young American abroad. There are stories of bus rides through dusty towns and glimpses of distant green hills -- perfect for exploring! -- and the joy of trying to pick up a little Arabic. He's reading of Judaism and Farsi and history --

Then his voice goes silent, then he's heard from again, and then silence.

Nick Berg will be commemorated at a memorial service at a local temple today. He will be buried in a private ceremony. He was 26 years old.

"When I look up at one of those 400-foot towers, I still see Berg climbing higher." Lorenz stops himself and starts again. "He had a great, great life spirit."

Peter Lu looks at a photo of Nick Berg, lower left, in their high school yearbook as he talks about the friend killed in Iraq."Nick was an explorer," says Peter Lu, a high school friend. "He went where no one else did. If there was a path, you could bet Berg wouldn't be on it."Nick Berg helps set up an engineering conference in Hershey, Pa., last year. He was a natural student who pursued a variety of subjects with zeal.Berg at a high school science fair with classmate Rebecca Hubbard.