Ten-year-old Annovis Ngo scrambled out of a stream and up some rocks, her voice breathy, her eyes shining.

"Oh, my gosh, there's so much over there," she said as she and a partner emerged brandishing a small net full of pebbles and water fauna. "There's a lot of those bug thingies!"

Another girl rushed up with a net containing a crayfish and a salamander and proudly placed them in a small plastic tank beside the stream.

"Hey, will you keep these as pets?" Alejandra Fernandez, 10, asked, as she squatted to watch the critters crawl in the dappled sunlight of the early fall day.

In their search for life in the stream, Annovis, Alejandra and their classmates from Carlin Springs Elementary School were taking part in a 38-year-old tradition in which Arlington County public school students are transported from their urban environment to rural Virginia for a wilder kind of education.

The adventure takes place 35 miles west of Arlington, just inside Fauquier County, at the Phoebe Hall Knipling Outdoor Laboratory, a 211-acre nature center in the Pond Mountains. There, far from the tract houses and traffic lights they're used to, students get to know a landscape with hickory oak forests, a spring-fed stream, a small lake and five miles of hiking trails.

The center is owned by the nonprofit Arlington Outdoor Education Association, which lets the Arlington public school system use the land. The system pays $164,000 a year for three full-time staff members and some maintenance and $66,000 for transportation; additional maintenance and other expenses are paid by the association, which takes in about $70,000 a year in donations and grants.

The lab, which is open to Arlington public school students and to Scout troops from the county, is named for Phoebe Hall Knipling, an Arlington teacher who was the first woman in Virginia to be a school district's science supervisor. She traveled around the state with high school science students on field study trips, and in the late 1960s she bought the land that became the outdoor lab.

"Things were getting more urban in Arlington," said Anthony Adamovich, the lab's assistant director. "She had a vision for Arlington kids."

Almost every school day, third-, fifth-, sixth- or seventh-grade students can be found taking in the tall trees and fresh air, with the crunch of gravel underfoot. A few brown shingle structures house the administrative center and animal lab, but most of the learning -- and sleeping and eating -- is done outdoors. The lab provides platform tents, and the students bring their own sleeping bags.

Sixth-graders come to do a survival game, in which they build shelters and fires and boil water to drink. High school science students come to collect data for their laboratory experiments; students in Washington-Lee's International Baccalaureate program use the lab to integrate their studies of chemistry, biology and environmental systems. There are also weeklong camps in the summer for fourth- through eighth-graders.

During the school year, most fifth-graders come for an overnight stay.

"It's the highlight of most fifth-grade classes, and very sought-after," said Constance Skelton, the school system's science supervisor, adding that because the center has room for only 60 students, each class is limited to one visit a year.

Much of the lab's curriculum is connected to the state Standards of Learning, said Neil Heinekamp, the lab's director. For example, third-graders, who are expected to know about six types of simple machines for the SOL exams, use pulleys to hoist a teacher into a tree and then examine how many third-graders it takes to perform such a feat. Fifth-graders, who study the difference between vascular and non-vascular plants, can see specimens up close. And high school students can study geomorphology (how land was formed) and analyze rocks.

But the SOLs are not the only reason they come.

"It's important that kids know about the world around them, and that's why we teach them about rocks, that's why we teach them about animals, that's why we teach them about plants," lab director Heinekamp said.

"I see kids that don't know how to light a match and I see kids that don't understand the concept that if they walk 10 feet into the woods, they will find more firewood for their fires than if they look on the road," he said. "They are missing some experiences that I think I had, experiences that would enrich the kids and make their lives easier and make them better listeners and thinkers."

Jay Jacobson, one of three Carlin Springs fifth-grade teachers whose classes visited the lab recently, said the excursion is a unique opportunity for his 13 students, most of whom are learning English as a second language. "Most of their parents are from Central America or elsewhere," he said, adding that overnight camps generally are not part of their culture. In addition, he said, many live in small apartments without yards. "Because the neighborhood's not good, most of these kids stay inside watching TV," he said. "They never get anything like this. They're so excited, and it's so good for them."

A few don't get to stay overnight because their parents are uncomfortable with the idea of their sleeping away from home. Sometimes the school system sends a bus to shuttle them home at night and back to the camp in the morning; if no bus is available, the children join other classes at school.

To Claudia Narvaez, 10, spending her first night away from home -- in a tent -- was exciting. "It's, like, so quiet," she said. "You can hear all the train noise, you can hear the owls, you can hear the wolves. And we can also see the stars and the moon."

Dhyamond Crenshaw, 10, didn't get the opportunity to miss her mom. Her mother, Dawn Crenshaw, 33, was one of the chaperones with the Carlin Springs group for the night -- and it wasn't her first time. She visited the lab with her fifth-grade classmates while a student at Randolph Elementary School in the early 1980s. Crenshaw had forgotten one thing from her first visit. "The nighttime was so cold," she said. Dhyamond grinned and threw her arms around her as if to warm her.

Mike Nardolilli, president of the Arlington Outdoor Education Association, said families like the Crenshaws are not unusual in their connection to the nature adventure.

"It's a real Arlington institution, when you span generations like that," he said, adding, "I remember one parent said it was the first time they saw a deer in the wild, which in those days was a lot more rare."

For some children, confronting the great outdoors can be fraught with worry. During the Carlin Springs visit, one boy slipped near the riverbed, then fretted about the muddy spot on his pants knee, worried about how his mother would react.

"I see the mud; it's just mud," Jacobson said comfortingly. "You know what that means, when you get muddy and wet? It just means that it was a little more fun, that's all." To make the boy feel better, he promised to help him rinse the pants clean.

After the night hikes, water investigations and lessons on weather, the children got an hour at the end for recreation. Some chose to boat, venturing into tippy canoes and shrieking as they paddled to the middle of the lake. Others pushed worms onto hooks and fished for sunfish.

"You can go down to the snake pit," said Adamovich, the assistant director, referring to an area below a dam that is marshy (but not actually a snake pit). "But take a partner, so if one of you falls into the quicksand down there, somebody can come up and tell us."

"Cool!" one child whispered.

A few kids hiked to the animal lab, home to a variety of creatures including a toad, a water snake and a box turtle with a missing leg ("I think an eagle bit it off or something," explained Douglas San Miguel, 10).

As two other children looked on, Elaine Khuu, 10, mothered a yellow corn snake named Emily that was silky to the touch. The snake wrapped itself around her arm and shoulder. Elaine explained that it was warming itself.

"Also, I figured out where they poop from," she added.

For their final activity before heading home, the kids roasted hot dogs on sticks over an open fire. As they screamed and ran around, Adamovich noted that children who are shy or have difficulty in a classroom environment often gain confidence and show hidden aspects of themselves at the outdoor lab.

"Sometimes they have problems in the classroom and they shine out here," he said. "So their teachers see them in a different light."

Michele Karnbach helps Kathy Patino, center, and Jassmine Wise analyze items scooped from a Fauquier stream. Fifth-graders Juan Ornelas, front, and Javiar Echeverria prepare to take to the water with Anthony Adamovich at the Arlington schools' nature center.Resource assistant Michele Karnbach guides fifth-graders from Carlin Springs Elementary School on a stream study.Diego Mariaca rolls around on a tent. The overnight outing 35 miles away from Arlington County is a highlight of the school year for many students.