"A stream," naturalist Rob Gibbs explains to a group of a dozen adults and at least twice as many kids who have come to Meadowside Nature Center in Rockville specifically to meet a stream, "is a rough place to live."

The tadpoles eat the microscopic organisms. The crustaceans eat the tadpoles, and the fish eat the crustaceans.

But a stream is a nice place to visit on a warm summer day, so armed with nets, seines, buckets and paper cups, the group (including a baby in a back pack) trudges down Sleepy Hollow Trail to the North Branch of Rock Creek. One 11-year-old boy carries a sort of mini-laundry bag.

"It's for collecting snakes," he explains. "I love snakes -- poisonous or non-poisonous. But with this crowd, we probably won't see any poisonous snakes."

Nevertheless, some people seem to decide right then and there not to go near the water.

"How is this water?" asks an anxious-looking father.

"Well, you wouldn't want to drink it," answer Gibbs. "But it's all right."

Designating a basin on the bank as the collection point, Gibbs passes out pictures of the critters to be found, and the search begins.

"What shall I do with this water strider?" asks a boy?"

"Put it in the tray and we'll talk about it later," replies Gibbs, recruiting another kid to carry one end of a large seine net. Before long, Gibbs and the kid have netted a small fish called a bluegill and are transfering it to a bucket.

"When you pick him up, make sure your hand is wet," instructs Gibbs. "Because that slime on him is important to him."

"Is is cold when you first get in?" asks a kid on the bank who's having trouble taking the plunge.

"Whoa, Daddy!" protests another kid who has taken the first step. "It's already up to my knees."

In a quiet pool at the side of the stream, a brother-and-sister team has scooped up a salamander, two crayfish, a tadpole and several minnows.

Meanwhile, one of the seine-net crews has caught a shiner and then a tiny fish with red gills that the kids label "the mystery fish."

"I think you guys might have good hunting if you just stay here by the falls," advises a father. "Let the fish come to you."

Downstream, a little girl has found a whole colony of crayfish under some rocks in shallow water, but is having trouble getting the critters into a bucket.

"Sort of grab them from behind," suggests Gibbs. Crayfish, Gibbs explains after the critters are safely in the bucket, go backwards by flipping the tail.

"It's like jet propulsion," only backwards, he says.

Some of the younger kids are more interested in the water than in its critters, and are happily wading and splashing.

"We should have weared bathing suits," says one, whose shorts are already waterlogged.

Gibbs is coaxing waders and critterhunters alike to gather on the bank to examine the catch, but some kids are scooping the water until the very last minute.

"I caught something -- no kidding!" yells a little girl holding up a net with a crayfish in it.

Another kid has nothing visible in his net, but is not to be outclassed.

"I caught some microscopic animals. Want to see them?" he challenges.

"I caught a leaf with my bare hands," boasts another boy.

On the bank, the catch looks impressive. There are daddy-longlegs in jars with holes in the lids, and an American toad in a dry bucket. In the basin of water there are half a dozen crayfish, lots of small water insects, some tadpoles, a few salamanders and a variety of tiny fish.

"Hope everybody in there likes each other," says Gibbs, but one of the fish, a kid reports, has already jumped out.

"Was it one of ours?" another kid anxiously inquires of his seining partner.

Gibbs plans to throw the critters back anyway, but first he wants to talk about them. You can tell a male crayfish from a female because the male has longer legs, he explains. The female's legs are shorter to make room for the egg sack. And when crayfish grow, they shed their skins -- like softshell crabs.

"Are these the kind of crayfish you eat?" asks a voracious adult.

"In Louisiana, they do," says Gibbs, making it clear he is not encouraging that kind of activity.

"He bit me!" shouts a child who has picked up a crayfish to examine the length of its legs. When the crayfish have been liberated, talk turns to the toad.

"If this was earlier in the season, you could hear him singing," wishes Gibbs. "His singing sounds sort of like a weapon from Star Wars."

A little girl makes a Star Wars weapon sound, and Gibbs shouts: "That's it! That's exactly what it sounds like."

After examining the gills of a salamander and pinning down the mystery fish as a red-gilled dace, the kids all want to go back into the water. This is all right with Gibbs, because he wants to gather plankton -- microscopic organisms -- in a special net so he can project them on a micro-projector back at the nature center.

"Plankton," he explains, "is the base of the food chain -- we'll just make a few sweeps."

When the water samples (which, he hopes, contain plankton) are put in plastic containers, Gibbs leads the group on a wade downstream. This time they're not going to gather critters but to learn more about the stream itself, to talk about ripples and shallows and pools. In the shallows, water is being cleaned on the rocks. Rapids put air into the water. What's the importance of a pool?

"Some fish can't stay in the rapids --" ventures a kid.

Yes, says Gibbs. Carp, for example, need the space, and there has to be someplace for breeding to take place.

At last and reluctantly, the stream-searchers step onto dry land and, sneakers sloshing, climb the trail back to the nature center. Gibbs puts some drops of stream water on glass slides and turns out the lights.

"Is it going to be a long movie?" asks a tired child.

Just then the star of the flick -- a microorganism -- slithers across the screen. Everybody cheers. This is the critter that feeds the tadpoles that feed the crustaceans that feed the fish that make a stream such an interesting place to visit.