Park ranger Steve Sarles sized up the gaggle of grade-school girls and decided to tease them a bit.

"Who's ready to go swimming?" he asked, and smiled when they all shouted "Me!" He added, "On the last water program I did, can you guess what the first thing we found was?" They shook their heads. "A big ol' water snake!"

"Oh, no, I don't want to go," 10-year-old Kim Gillespie said, but her extravagant shudder and rolling eyes suggested otherwise. It was time for a jaunt to Carter's pond in Prince William Forest Park, a national preserve about an hour south from Washington.

Sarles and company proceeded to plumb the mysteries of Carter's Pond, a 1930s-vintage man-made pool named, presumably, for a farmer called Carter. Splashing about at water's edge, they sought spiders, frogs, fish, beavers, bugs and other "critters," as Sarles called them, that thrive in the wet.

"I'll tell you right now," he said, "we don't have any water snakes in the park that are poisonous. So if you get bit, don't go jumping up and down, screaming and thinking you're gonna die."

The group, a score of kids and a few parents from Maryland and Virginia, basked in a summerlike sun as Sarles spoke, sitting amid beaver-gnawed tree limbs and budding wildflowers beside the water.

Every so often his running commentary -- "Anything we find alive, we want it to be alive when we leave" -- was interrupted by the flight of a nice-sized bumblebee. Lots of shrieks and giggles as the girls squirmed out of its path. "Anybody sitting in a bee nest back there?" the ranger wondered.

The afternoon's business was to scoop up critters in plastic containers that Sarles had brought along, examine them closely, then release them unscathed. Sarles, meanwhile, would conduct an experiment to determine the water's oxygen content.

First, though, he gave them a primer on pond life, tackling such issues as surface tension, water temperature, why ice floats and where critters hide. "But I'm not gonna give you hints on where to look," he said. "Okay, let's go."

The girls swarmed around the ranger as he handed out containers, and ran gleefully for the water. Some stepped cautiously, getting only their sneakers wet -- "It's cold," one shouted -- while others more bravely waded in up to their knees. Kim Gillespie and Susan Anderson, 12, made the day's first catch, and held it out to Sarles in a jar.

"That's a cricket frog, I think," he said. "If we heard him at night, we'd know that real quick. Careful now, this guy has legs that are good for jumping. He breathes through his skin. Ever hear how you shouldn't paint animals? That's why."

Like a tightrope walker, Pam Snyder, 14, moved out along a downed branch, grasping a friend's hand for balance. Bending over, she grabbed first for a salamander lurking near the surface, then, barehanded, for a tiny fish. Then she presented her jar.

"All I got was a spider," she said dolefully.

"Hey, everybody, here's a water spider," Sarles announced, holding it up. "Everybody have a chance to see it? Anybody know if a spider's an insect?"

A squadron of kids ran up, out of breath. "We found a salamander!" their leader said. Sarles took the jar and looked it over.

"We call this guy a salamander," he said, "but I think he's a newt. He's a young newt, I'd say. When he gets older, he'll move away from the pond and live in the forest."

No beavers appeared, but Sarles pulled a few small branches out of the water. They'd been stripped clean of bark -- winter food for the industrious chewers.

Then he performed his oxygen test. He took a vial of pond water, rummaged through his kit, and added a strange chemical. "This should turn yellow if the oxygen level's good." It did. Then he took an eyedropper filled with another chemical and counted as he squeezed. By the seventh drop, the vial turned clear again, meaning the water contained 700 parts of oxygen per million parts of water: a high content, Sarles said. Then it was back to the pond.

He crouched near the water and started turning over branches and rocks. "Somebody hold my hat," he said, and the girls whisked it off his head and tossed it to and fro. "Be careful, those are expensive," Sarles said with an icy grin. They held the hat teasingly over the water. "The last kid who played with my hat, he really got it," Sarles cajoled, as he tried to maintain his composure. "I want you to know, I've killed people for less than that."

But the girls just laughed. TO THE POND "Life in the Pond" is a regular offering at Prince William Forest Park in Virginia, off the Route 619 Exit of I-95. To find out about that and other free programs, call 703/221-2104.