FOURTH OF JULY FIREWORKS were the big event last night. Fireworks aren’t sold in Massachusetts, but George and Haley’s parents bought a box in nearby New Hampshire, where they’re legal. Once it was dark, we set up the rockets and Roman candles along a bench on the lawn and fired them off into the night.

The children love to light sparklers and run around the lawn with them, trailing streams of sparks. While George was running with sparklers in both hands, he tripped and fell flat. But he bounced right up again with a laugh, like it was no big deal.

The final display was called a fountain, which sent great showers of colored sparks about ten feet up into the air for several minutes. Julie said it looked kind of like a volcano—only there was no magma and it didn’t leave a pile of lava behind.

THIS MORNING, HALEY AND JULIE decided to go down to the beach for a swim. Afterward they sat on the seawall in the warm sun and talked. Haley knew about our rocket trip into space to look at the whole Earth because Tom had showed her his photos. She also knew that Julie is the Water Woman, so while they sat looking out over the bay, she asked why Julie was so interested in the oceans.

Julie filled her in. About how the oceans cover three-quarters of the Earth. About how heat from the sun turns the water on top of the ocean into a gas—water vapor—that goes up into the air and turns into clouds when it cools. About how winds blow those clouds over the land, where the water vapor cools even more and then falls to the ground again as rain.

Haley knew about rain, of course, but she didn’t realize that it originally came from the oceans. Like her cousins, she then wondered why the water in the ocean is salty but it is fresh water when it falls down as rain.

I arrive just as Julie is explaining about how the salt stays behind when ocean water turns into water vapor. I’m wearing an old pair of tan shorts, a t-shirt, and a Red Sox baseball cap.

“Hi, Grandpa,” Julie says. “I was just telling Haley about the water cycle, and how the rain that falls on the land actually comes from the ocean.”

“And Julie told me that she’s the Water Woman,” Haley chimes in, “because she knows so much about the oceans and how they keep us all alive.”

I sit down beside them. “That’s right.” I nod toward Julie. “She’s the ocean part of our Gaia team. Did you tell Haley about Gaia, Julie?”

“Not yet,” Julie says. “But I can tell her now, and you can help if I miss something. I remember that Gaia is an old word that means living Earth.”

Haley frowns. “Do you mean the Earth keeps us alive?” she asks.

“That’s part of it.” Julie nods encouragingly. “It’s like with the water cycle. The oceans work together with the sun to send water up into the air, and it blows over the land to give us fresh water we drink. The rain also waters the plants we eat. So the Earth is working to keep us alive.” She pauses briefly. “But you can also say that the Earth itself is alive—that the whole Earth is a living thing.”

“That’s the part I don’t get,” Haley says, looking at me. “How can the whole world be alive? It’s just dirt and water with air all around it.”

I want Julie to try to explain the hard part. “What do you think?” I prompt her.

Julie jumps off the wall and looks out over the bay as she thinks. Then she turns back to us. “So, Haley,” she says, “let’s start with you and me. We’re alive, right?”

Haley sighs and rolls her eyes. “Yesss,” she says. “Soooo?”

“So, we’re alive because we breathe oxygen in the air, and we eat food that gives us energy to do things. We use certain parts of our body to breathe, okay? They’re called our lungs.” Julie takes a deep breath to show Haley how her chest goes out and in.

Haley doesn’t seem impressed yet by Julie’s explanation. She sticks out her bare legs and inspects a scar on one kneecap. “I know about my lungs,” she says, tossing her head. “But the Earth doesn’t have lungs.”

“No, that’s true,” Julie admits. “But the oceans work kind of like our lungs. They take in sunlight and give off water vapor that turns into clouds. Isn’t that kind of like breathing?”

The skin on Haley’s nose is peeling from too much sun. She scrapes at it with her thumb as she thinks about this idea. “Kind of, I guess,” she finally says.

Julie’s on a roll now. The idea of breathing is a good way to talk about being alive.

“Tom is the Gas Guy. He taught us another way the Earth breathes,” she says. “The trees and other plants all over the land breathe in a gas called carbon dioxide from the air. Carbon dioxide has oxygen in it. Then they breathe out the oxygen that we need. So that’s another kind of breathing, sort of like what we do.”

“Okay,” Haley says, wiggling her toes in the sand. “I guess you can say that the Earth breathes, kind of like us. So maybe it is alive in its own way. What was that word you used before … Gamma?”

“Gaia,” I say and spell it. “G-A-I-A. Gaia.”

Haley looks pleased. “I get the living part now,” she says, hopping down off the wall. “The Earth is living because it breathes. And it keeps us breathing!”

A new idea occurs to her. “Hey, Julie— if you’re Water Woman, then I want to be Gaia Girl!”

Julie looks at me proudly. “Okay, Haley, you can be Gaia Girl.” She points to some rocks she and Haley have gathered on the beach.

“Gramps, we want to ask you about these rocks we collected. We picked ones that look the most different from each other. But how come they look so different when they all come from the same place?”

“Actually they didn’t all come from the same place,” I answer.

Both girls look at me like I’m nuts. “Sure they did!” Julie insists. “We picked them all up this morning, right from our own beach!”

“I know you did.” I laugh. “But they weren’t made here—not like that volcano was making rock on Hawaii in the video we watched yesterday. These rocks were brought here from somewhere else. Any idea what brought them?”

These rocks were carried to our beach on Cape Cod by a glacier that pushed down from Canada thousands of years ago, during an Ice Age.

The girls look at each other and shrug. “Maybe the water somehow?” Haley suggests.

“That’s a good guess. Water did bring them, but the water was frozen, not liquid.”

“Ice?” Julie wonders. “The rocks were brought here by ice?”

“That’s correct.” I pick up one of the rocks and admire it. “Do you know what a glacier is?”

“Isn’t that the ice way up by the North Pole?” Julie asks. “I think Tom learned about it in science class.”

“That’s right. Glaciers are incredibly thick sheets of ice. Thousands of years ago it was much colder here. A glacier up in northern Canada grew bigger and bigger as the snow fell on it and the ocean water froze. That glacier was two or three miles thick, and the front part moved as it grew. It inched its way south like a giant bulldozer, carrying rocks and dirt with it.”

Haley’s eyes grow big as she imagines ice several miles thick. “You mean it was like a big high mountain of ice creeping across the land?”

“Right. The front of it was moving slowly, and everything in its way must have gotten pushed along ahead of it.”

“Whoa!” Julie picks up one of the rocks and turns it around in her hands. “So this rock was probably scooped up from somewhere up north and then dropped here by that glacier.”

“That’s just what happened, more than twenty thousand years ago. Around that time the Earth’s temperature began to warm up again, and the atmosphere, too. So the glacier stopped growing and started to melt. As it melted, it got smaller, and these rocks were left behind right here on the Cape.”

“Were there people living here when the ice came?” Haley asks, shuddering a little. “It must have been cold and scary when that frozen mountain came creeping along.”

“No people,” I say. “But probably lots of big animals, some even as big as elephants. They moved away to the south as the ice came crunching along from the north.” I point south across the bay toward the town of Chatham.

“I wonder if you could hear the ice mountain coming,” Julie muses, looking northward. “Of course, if it was two miles high you could see it from a long ways away!”

While we were sitting on the seawall talking, the tide in the bay was creeping up the beach, until finally water was washing around our dangling feet. As is it inches in over the sun-drenched sand flats, the clear water warms almost to bathtub temperature, and Julie and Haley decide to go for another swim. I stay on the wall, watching them frolic and enjoying the warm sun on my face.

After the girls come back from swimming, we wander farther down the beach to where Tom and George have been busy building a sand castle. By now the tide has turned and is beginning to flow out of the bay again. Around the castle they’ve dug a moat, and behind the moat they built a high sand wall to protect the castle from the incoming tide.

So far these defenses have saved the castle, and the boys are congratulating each other. While Tom makes small repairs, George describes how their design for tidal protection worked. Besides the main inner wall, they also built a wall on the bay side of the moat to slow down the wind-blown waves before they reached the moat. But that wasn’t all.

“We knew that after the tide came in and surrounded our castle, it would go out again,” George explains proudly. “So we built an outer wall behind the castle too, on the land side. When the tide started to go out, that protected the main wall from the water flowing back to the bay.”

I’m very interested in the details of the castle and the ways George and Tom shielded it from the water. I remember building sand castles a lot like this one when I was their age, using some of the same ways to keep tidal water from breaking down the castle walls.

By now the tide has dropped enough that their castle is safe. The moat is still full of water, but a little stream is running out of the moat, digging a channel in the sand as it flows down into the ebbing tide. Right at the water line, the sand carried by the stream has piled up in a fan shape just below the water’s surface.

I’ve always been interested in streams and rivers. I think it’s because I love to fish, and some kinds of fish live there. “Hey, Tom,” I say, “you even have a nice little stream running out of your moat.”

Tom looks up from his wall-fixing work to see. “Oh, yah,” he grunts, “pretty cool,” and returns to his wall repair.

But George walks over and crouches down to look more closely at the running water. “Gramps, if this was a real city, say like Boston, that would be a big river,” he says.

I like George’s imagination. “It sure would,” I respond. “And look at all the sand it’s carrying out to sea.” I point to the fan-shaped mound piling up at the mouth of the “river.”

“Do you know what that’s called?”

Water flowing down the beach as the tide goes out carries sand with it that forms a delta.

George answers confidently. “Yup. It’s a delta. In school we learned about the Mississippi Delta, and how the oil from that oil spill in the Gulf of Mexico was messing up the birds and other things that live there.”

I’m impressed. “Good for you. Think of all the dirt and stones and stuff that the Mississippi River carries for thousands of miles—from the hills of Minnesota all the way down to the Gulf of Mexico.”

Now Tom is interested. “It doesn’t all come from Minnesota,” he points out. “You’ve got the Ohio River carrying stuff from the Appalachian Mountains west into the Mississippi. And the Missouri River runs from the Rocky Mountains all the way east and ends up in the Mississippi, too.”

“True,” I respond, nodding . “No wonder the delta at the mouth of the Mississippi is so big.”

Julie has been listening, thinking about how water rises up from the oceans to form clouds that blow over the land and drop rain or snow. She has an idea about where all that rain goes.

“Grandpa, does that mean all the rain that falls on the middle of America gets carried to the Mississippi River by streams and rivers – and then it ends up in the Gulf of Mexico?”

“That’s right,” I answer. “You’re describing a water-shed. You can picture it like a giant funnel cut in half, and the Mississippi River is the funnel’s spout. All the water or melting snow on the east side of the Rockies, the south side of the hills in Minnesota, and the west side of the Appalachians gets funneled into the Mississippi and carried down to the Gulf. It’s part of the water cycle you taught us about earlier.”

Haley’s eyes light up. “So the water in the rivers is kind of like the ice in the glaciers?” she offers shyly. “As it moves along, it picks up rocks and sand and things and carries them to the ocean. But it drops them at the edge before it gets mixed with the sea water.”

“That’s exactly what happens,” I crouch down and look closely at the little delta formed by Tom’s baby river. “That’s how a delta is made.”

“Watershed, watershed, watershed …” George is mumbling. “I know that an umbrella sheds water. So a watershed must be land that sheds the water that falls on it.”

Julie picks up the idea and extends it. “When the water runs downhill, it joins up with more water running down another part of the hill. Pretty soon it’s a little stream. Then a bunch of little streams may come together to make a big stream, and the big streams join to make a river. All the water that ends up together in a stream or a river is coming from the same watershed, I guess.”

Tom has finished his sand castle work and joined us at the water’s edge. Together we watch the little delta form and reform with sand carried by the tiny stream, as it flows from the moat around his sand city. “Gramps,” Tom says, “remember back in June, when I told you and Julie all about the carbon cycle?”

“Sure, I remember,” I say. “You taught us about photosynthesis, and how carbon from the carbon dioxide in the air becomes food for plants.”

“Right!” says Tom. “We also talked about how sometimes when they die, trees and other plants get covered up by dirt and sand, and after millions of years all the weight of that stuff squishes them into oil and gas.” He stands and brushes wet sand off his knees.

“I bet that delta at the mouth of the Mississippi River is covering up old plants and squeezing them into oil!”

George has been listening carefully, and when Tom mentions oil, he joins in. “That must be right,” he says, “because they have all those oil platforms in the Gulf of Mexico. The one that blew up and spilled all that oil into the water wasn’t so far from the Mississippi delta.”

Our stomachs are telling us it’s almost dinnertime. The girls run ahead to load their special rocks into a beach bag, and we head back to the house, Tom and George still talking about watersheds and deltas.

These oil wells pump up oil that was made millions of years ago from deep within the sea floor.

AFTER DESSERT, Tom and George head for my office, and Tom types Mississippi River watershed into the laptop’s search engine. One of the resulting pictures shows the whole watershed, with all the rivers that carry water from east and west and north into the “Father of Waters,” as the Native Americans called the Mississippi. It also shows the mountains where the snow melts and clouds drop the rain that flows down into those rivers.

The Mississippi River watershed drains the water that falls on all or part of 31 states and two Canadian provinces.

After washing the dishes, I join the boys, and Julie and Haley come in a bit later. Looking at the watershed map, George says, “It does kind of look like a funnel. All the water comes together in one narrow place at the bottom.”

“I found another map that shows the boundaries of the watershed,” Tom says, clicking on another image. “You can see how the watershed covers the whole middle part of America.”

Boundaries of the Mississippi Watershed

“That’s the state of Louisiana, down at the bottom where the big river runs into the salt water,” I observe. “The Mississippi delta is a pretty amazing place. It’s full of marshes where birds and fish live. Try googling pictures of Mississippi Delta.”

Pictures of the Mississippi River Delta

Satellite photo of the Mississippi delta. The green areas are fields and marshlands.

The delta is home to many different kinds of animals. These are pelicans.

One picture, taken from space, shows the land formation where the river water spreads out and empties into the Gulf. Another shows the marshland where so many birds live. It looks sort of like the marshes here around Pleasant Bay. And on a canal in the marshes, a photographer found a flock of beautiful pelicans.

After waiting patiently for her turn at the computer, Julie searches for pictures of how rocks are made. She soon finds one she has to share.

The Rock Cycle
Rivers and streams carry rocks and soil into the ocean. Over thousands of years they form sedimentary rock. When the Earth’s plates bump into each other, sedimentary rock may be heated and harden into metamorphic rock. If one plate slides under another, sedimentary or metamorphic rock melts and pushes to the surface through volcanoes. This rock is called igneous. Rain and snow eventually wear rocks down, and the cycle begins again.

“Look, Gramps,” she says excitedly, “it’s another cycle, like my water cycle! Only this is a rock cycle.”

“Very cool,” I look over her shoulder. “How does it work?”

“You start up in the mountains.” Julie points to the graphic. “Let’s say it rains up there, and the water runs down the watershed—there, with the river in it.”

“And the water carries all the rocks and sand and stuff, right?” adds Haley.

“Yup.” Julie enlarges the picture so we can see the rivers better. “That stuff all piles up at the mouth of the rivers. That’s what they call erosion and transport, because everything gets transported by the water. When it gets into the ocean, it sinks to the bottom and gets squished into rock. That kind of rock is called sedimentary rock.”

“Were any of the rocks we found on the beach that kind of rock?” Haley wants to know.

“I don’t think so,” I reply. “Your rocks looked like they had been twisted and bent.”

“That’s because they had been heated up, and they were under a lot of pressure,” Julie explains. “Ours are a kind of rock called metamorphic. It’s the next one in the picture.” She points to the word on the screen.

Tom and George left when the girls took over the laptop, but now they’ve wandered back in as the talk got excited.

George notices something in the picture that reminds him of what we learned about the movement of the Earth’s plates. He says, “I bet metamorphic rock is the kind that gets pushed up into mountains when two of the Earth’s plates bump into one another, like you were saying the other day, Gramps.”

“Right!” says Julie excitedly, pointing to the bottom of the picture. “See, one plate is pushing under the other plate. That makes heat, which melts sedimentary rock into metamorphic rock!”

She goes on, “So after the metamorphic rock gets pushed up into mountains, rain comes along and wears the mountains down again, over thousands of years. The sand and stones from those crumbling mountains wash down the rivers into the sea, and the whole thing starts over again. It’s a cycle!”

George doesn’t want us to forget his favorite source of rocks. “The picture also shows a volcano,” he observes. “See—there’s the magma that shoots up from underground and builds a mountain out of lava.

“And there’s another name for that third kind of rock,” I add. “It’s called igneous. Sometimes we find that kind of rock on our beach, too, carried there by the glaciers thousands of years ago.”

Pretty soon it’s time for Haley and George to walk back to the house where they’re staying with their parents. Later, while Tom gets ready for bed, I stop by to check on him and Julie. “I’m surprised by how much the land is moving and changing,” he says, stretching and yawning.

“It feels like it is solid and stays in one place. But it’s actually big plates floating around on a soft mantle. New mountains pop up from volcanoes, and old mountains get worn down by the rain. Deltas are formed where rivers run into the sea. It’s all pretty amazing.”

Julie is listening from the bathroom, where she’s brushing her teeth. “Yup,” she says between strokes. “And I think all the cycles are really cool. There’s my water cycle, of course, and the rock cycle, and Tom’s poop cycle, right?”

“Right,” Tom says sleepily, pulling on his pajama top. “And don’t forget the carbon cycle. Maybe Gaia is like one big cycle with lots of different wheels in it. Each of the wheels is a little cycle that’s part of the big cycle. And they all connect somehow.”

Julie sticks her head in the door. “Four wheels,” she says. “That’s not a unicycle, and it’s not a bicycle, and it’s not a tricycle. We can call it a Gaia-cycle!”

Tom climbs into bed and pulls the sheet up under his chin. “I like that idea,” he says with another huge yawn. “I can imagine the living Earth as a globe with skinny arms and legs, riding along on a big four-wheeled cycle. The planet has a big smile on its face, and it’s whistling …” His voice begins to trail off as sleep creeps in.

“Goodnight, Tom.”

“Goodnight, Gramps.”