OUR SAILING TRIP ACROSS THE BAY yesterday was fun. We had nice wind behind us on the way over to the beach. It pushed us along quite quickly, but didn’t make the sailboat tip too much. Tom has become a good sailor over the past couple of summers, and he steered the boat the whole way. Julie helped me handle the sail.

“It’s so cool that we can go this fast with nothing but wind power!” Tom called out, hanging on tight to the tiller as a big puff made us surge ahead.

Once we got to the beach we ate our sandwiches, and Julie and Tom played in the surf. We didn’t have boogie boards with us, but they body-surfed on the 
incoming waves. On the way home we sailed over to a big seal colony that lives in deep water, close to where the bay flows out into the ocean. Most of the seals were resting up on the nearby sandbar.

When we sailed by very slowly, six or seven of the younger ones swam over to check us out. They dove 
under the boat and popped up on the other side. Or sometimes they swam ahead of the boat but under water. We could see their shadows against the sandy bottom of the bay as they zigzagged back and forth. Then two of them poked their heads out of the water and stared at us as we sailed past.

“Hey, seals!” called Julie, waving. To us she said, 
“I wonder what they think we’re doing out here?”

“Having lunch and breathing out carbon dioxide— same as them!” Tom replied with a laugh.

This little guy popped up right beside our sailboat.

LAST NIGHT TOM ASKED to use my laptop to find out more about plants and what happens when they die. I could hear him, on his bed up in the loft, looking stuff up on the Internet and taking notes. This morning he is surfing the Web in the kitchen while we make breakfast. It’s raining quite hard, so we’re all happy to be inside.

When we’re done, I ask Tom if he’s ready to share his latest discoveries. “Yup,” he says. “The whole story about plants living and dying is pretty interesting. And I found out about coal and oil, too.”

“Excellent!” I say. “Where do you want to start?”

Tom points out the window at a big oak tree in the woods behind the house. “Let’s start out there,” he says.

Julie gets up from the table and looks out the window. “Where?” she asks, looking confused.

“Start with the leaves on that tree,” Tom says. “What’ll happen to them in the fall, when it gets cold?”

“They’ll fall to the ground.”

“Right, and then after a while the earthworms and caterpillars and bugs will eat them for food. You know what happens then, don’t you?” Tom giggles a little.

“The worm poops, I guess,” Julie says, wrinkling her nose.

“Yes, and what comes out is good fertilizer. It’s like when cows or deer poop—their manure is good plant food too. It makes plants grow better.”

Earthworms eat leaves and turn them into plant food.

Julie thinks for a minute. “I think I see,” she says. “So the tree that dropped its leaves can use the worm poop to make next year’s leaves?”

“Exactly right!” I say. “Julie, do you know what a 
cycle is?”

Julie looks over at me. “A cycle … is it like a bicycle? Or a unicycle?”

“A cycle must be a wheel,” Tom says. “Like, a bicycle is two wheels and a unicycle is one wheel.”

“That’s right,” I confirm. “A cycle is a kind of wheel that goes round and round. Our planet, Gaia, has four or five really important cycles—wheels that go round and round to help her do her work.”

“Actually, that’s just what I was about to explain,” Tom says impatiently. “There’s this cycle that people call the nutrient cycle. But I call it the worm-poop cycle!” 
He grins. “When plants die, they usually go through the worm-poop cycle and then they feed more plants.”

“Okay,” says Julie. “When a tree drops its leaves, worms and bugs eat the leaves. They poop out plant food. What happens next?”

“Well, when it rains, the plant food soaks into the ground,” Tom begins. “The tree takes in the food and water through its roots. The food helps the plant make more leaves. Then those leaves fall off the tree, and the cycle starts again.”

He reaches for the computer and pulls up a website. “Here, I’ll show you a picture of the whole process.”

“See, the tree takes in the food and water through its roots. The food helps the plant make more leaves. Then those leaves fall off the tree, and the cycle starts again.”

Julie looks closely at the illustration. “So the worms are what they call decomposers in the picture. They turn the leaves into something called organic matter. That must be plant food. What about the deer? Does it poop too?”

The Nutrient Cycle
When plants die or are eaten by animals, they are turned into soil that is food for new plants.

Tom grins. “Sure. And the deer poop also fertilizes the tree and other plants. Take a look at this other picture I found of the different things that can happen with plants out in nature.”

How Plants Become Food for Animals and Other Plants
Most plants are producers, and most animals are consumers. Mushrooms play a special role.

Julie and I peer at the illustration on the screen, which is labeled with a bunch of words he hasn’t mentioned yet. So I ask, “What’s the deal with ‘producers’ and ‘consumers’?”

“You start with the plants,” Tom explains. “Like when we talked about photosynthesis yesterday? That’s where plants take carbon out of the carbon dioxide gas in the air, and use it to grow? What happens is, plants produce the carbon using the sun’s energy, so they’re called producers. That tree outside is a plant, so it’s 
a carbon producer.”

“Got it!” Julie says, pointing to the illustration. “Then that chipmunk eats the grass, which is the same as consuming it. So the chipmunk is a consumer?”

“Yup.” Tom nods and points to a big bird—a hawk—in the picture. “If the hawk eats the chipmunk, then it’s a consumer too. It consumes the carbon from the grass, but only after the chipmunk has consumed it first. That’s why the hawk is called a secondary consumer.”

I point to a mushroom at the bottom of the picture. “Why is the mushroom called a decomposer?”

“Oh, that,” Tom answers. “It works kind of like the earthworm. But instead of eating the leaves or the rotting log, the mushroom gives off a liquid that sort of melts the leaf and turns it into soil, see?” He points to where the arrow goes from the mushroom to a pile of rich soil.

“Let’s get back to the poop,” Julie insists. “Those other animals—the chipmunk and the hawk—they poop too, right? And that makes the soil better for growing the trees and the grass.”

“Correct,” Tom responds. “I call it the poop cycle 
because the animals and worms are eating grass and leaves and pooping it out to fertilize living plants. And mushrooms help too. It’s kind of like what happens in our stomachs when we eat food. The liquids in there dissolve the food into tiny pieces so our blood can carry it to other parts of our body.”

When Tom told us about leaves falling off the trees, he said that they usually go through the poop 
cycle. He didn’t say that always happens. “Tom,” I ask, “is it true that sometimes plants don’t go through the poop cycle when they die?”

Tom’s eyes light up. “That’s right!” he says, spinning around in his chair. “Let me show you a bigger picture.” He clicks on a page he has saved. We gather around to look.

Pointing to the upper left of the picture, Tom says, “Start here. You see the sun, and then you see where photosynthesis happens. Leaves on the tree use energy from the sun to turn the carbon dioxide into carbon, for plant food, and oxygen for us to breathe. See where it says CO2?”

The Carbon Dioxide Cycle
Plants use energy from the sun to turn CO2 gas in the atmosphere into the oxygen we breathe and carbon to help us grow. People and other animals breathe out oxygen and CO2 so that they can be used again by plants.

“What’s see-oh-two?” asks Julie.

“It’s just a shorter way of saying carbon dioxide,” Tom replies. “The C stands for carbon and the O stands for oxygen.”

Then he continues. “Now, let’s say that tree dies. Usually it rots, and bugs and worms and mushrooms turn it back into food for other trees.

Julie is getting impatient. “You already told us that part,” she says, taking an apple from a bowl on the table. “Gramps, is it okay if I eat this apple?”

“Sure,” I say. Tom picks one too, and they both take big crunchy bites. After they have munched for a minute, Tom returns to the laptop. He points to the bottom of the picture, where an arrow points down into the ground, and a label says fossils and fossil fuels.

“See,” he says, “millions of years ago some plants and even some dead animals didn’t go through the poop cycle. Instead, they got covered up with mud or water, like in a swamp or from a flood.”

Juice from her apple is dripping down Julie’s chin, so she gets a paper towel. Between bites she asks, “What does that have to do with fossils—and what are fossil 
fuels, anyway?”

“Do you know what a fossil is?” I ask her. “Sure,” she answers. “A fossil is an animal or a plant that died and somehow turned into rock. In school they’ve shown us rocks with little leaves and snails and things in them.”

“Wait!” says Tom. “You’re getting ahead of my story. Let me show you what the world looked like a 
gazillion years ago.” He clicks on a National Geographic website about the history of the Earth, and finds a part describing the world about 300 million years ago. “Here are some pictures of what it looked like back then, when the land was really swampy and covered with huge trees and plants.”

Julie looks over at the screen. “Okay. Big plants living in swamps. So what?” She takes another big bite of her apple.

Tom won’t be rushed by his sister. “So, way back then, when the trees and other plants died, lots of them fell into the water and sank down into the mud.”

Millions of years ago, the land was covered with trees that lived in wet, swampy places.

Julie finishes her apple and tosses the core in the trash. “Trees in the muck. Sounds kind of yucky. What happened next?”

“Remember, this took thousands of years,” Tom 
reminds us. “Over all those years, rain fell on the mountains and washed dirt and rocks down into the swamps. After a while the dead trees and other plants got covered up by the rocks and dirt. The weight of all that stuff squished them more and more, squeezing out all the water. Finally nothing was left from the plants, except the carbon.

How Plants Turn into Coal
When trees and other plants get buried under dirt and rock for millions of years, they heat up and very slowly turn into coal.

Julie leans over and looks carefully at the picture now on the screen. “I get it,” she says after a minute. “When you squish plants long enough and hard enough, they turn into coal. But what exactly is coal?”

“Coal is a kind of rock made out of carbon,” I 
explain. “If you heat it up, it will burn—kind of like the wood it’s made from, only it burns longer and hotter.”

“So how about the oil? Is that just melted coal?”

“Not exactly. I looked that up too,” says Tom, clicking over to another website. “Oil was made when little sea animals and plants died, and sank to the bottom of the ocean. Like the trees on land, they also got covered by more and more sand and mud over thousands and thousands of years. The sea animals and plants got squished, like with the coal. It got really hot down there under the weight of all that sand, and the heat helped turn the dead animals and plants into oil.”

“So the difference between making coal and making oil was that coal came from plants and animals buried on land, and oil came from when they were buried under the ocean. Is that right?” I ask Tom. He nods.

Julie still has a question. “Okay, so that’s how coal and oil were made millions of years ago. But why are they called fossil fuels? They aren’t made of fossils, are they?”

I’m not sure of the answer. “Maybe it’s because the fossils we find in really old rocks help us understand what kinds of plants and animals got squished up long ago and turned into coal and oil,” I suggest. “Let’s take a look at some fossils.” I type pictures of fossils into Google, and lots of images pop up. “Look here,” I say, “you can see what’s left of a very old plant, and a horseshoe crab, and even a fish. They’ve all been turned to stone by the weight of the earth on top of them and the heat coming up from inside the Earth.”

Tom comes around the table to look. “So those are some of the plants and animals that got turned into oil millions of years ago?”

“I think so,” I answer.

Julie has a new thought. “Fuel is something you burn, right? Like we burn oil in the furnace down in our cellar to heat up the house. Or wood in the fireplace?”

“That’s right. And coal is burned in big power 
stations to make enough heat to warm all the houses and factories in a whole city. And to make electricity, too. Tom, can you take us back to that picture you showed us a few minutes ago?”

Tom clicks on an icon. “Here it is again. That’s a cycle too, you can see— kind of like the poop cycle. And it begins the same way, with photosynthesis.”

“Tom, let me go around the cycle.” Julie nudges him aside and gets up close to the screen. “I start with the sunlight, and that makes photosynthesis happen in the leaves of the tree, right?”

We both nod, and Julie continues. “Trees take carbon dioxide gas out of the air. Then they get the carbon out of it, and that gets stored in the tree. When the leaves fall off, and worms and bugs eat them, you get the poop cycle.

Tom tries to interrupt, but Julie pushes ahead, 
following the cycle by talking it through. “Millions of years ago, some of the dead plants and animals got buried in the land and under the ocean. That’s what made the fossils and the fossil fuels at the bottom of the picture. Then we come around to all this respiration.” She looks up, uncertain. “Is that like breathing?”

“That’s just what I was going to tell you!!” Tom 
exclaims, irritated that Julie is telling “his” story. “When animals or people like us respire, we’re breathing—we bring oxygen into our bodies and breathe out carbon dioxide. When we eat plants and meat, carbon gets into our bodies. When we breathe out CO2, some carbon gets back out into the atmosphere again.”

“It says here that plants do respiration too,” Julie points out. “Does that mean plants breathe like we do?”

Tom glances over at me, so I pick up the story. “Plants do their own kind of breathing,” I explain. “During photosynthesis they take in carbon dioxide from the air and send oxygen back out into the air, so that’s a kind of breathing. But it can only happen in the daytime, when the sun is shining and providing energy.

“Respiration is different,” I go on. “When plants respire—which is happening all day and all night—they mix oxygen with some of their carbon and send them back out into the air as carbon dioxide. But plants hold onto some of the carbon they take in, because they need it to grow bigger.”

“Got it!” Julie turns back to the screen. “I can see that the tree is respiring through its leaves and through its roots. Then the carbon dioxide coming out of plants cycles around, and the plants pick it up again for photosynthesis.”

“That’s right,” I say, giving Julie a big smile. “Around and around the cycle goes. Tom, that picture is really helpful. What about where it says auto and factory emissions? How do they fit into the cycle?”

“Good question, Gramps.” Tom closes the picture of the CO2 cycle and opens another one that shows two cycles, one inside the other.

Julie and I lean over his shoulder and stare at the screen. “What are we seeing here, Tom?” I ask. “Walk us through it.”

The “Extra Carbon” Cycle
The green arrows show the natural carbon cycle. The dark gray arrows show the extra carbon cycle, which happens when we burn gas, oil, and coal.

“Let’s start with the little green cycle,” he says, pointing at it. “That’s what I call the natural carbon cycle. The tree takes carbon out of the air and puts oxygen into the air. And the tree is storing some of the carbon in its trunk and branches. But it also returns carbon to the
atmosphere by giving off carbon dioxide—that’s the respiration part.”

“How about the big dark gray cycle? What do you call that?” Julie wants to know.

“I call that the extra carbon cycle, because it shows how us humans are adding extra carbon to the air. We do that by burning coal and oil to heat our homes and power our cars and things. All that carbon that was stored deep down in the ground, as coal and oil, is being sent out into the atmosphere.”

Julie studies the picture. “I see a lawn mower. And I see smoke coming out of the chimney of the house. So that’s the burning part.” She stops and scans the picture again, tracing the cycle with a finger.

“I see the difference now. The carbon still goes from the air through the trees into the ground. That’s the natural part. But then we get it out of the ground and burn it, which puts the buried carbon back into the air. That’s the part of the circle that humans do. We burn the stuff in our houses and cars and factories.” Julie wrinkles her forehead, looking a bit worried.

“Yup. We’re putting a lot more carbon into the air that way.” Tom has found a piece of paper and is drawing lots of trees on it. “That means we need lots of trees to take the carbon back out of the air again. They help keep the atmosphere from warming up too much, right?”

“Absolutely,” I say. “They do help a whole lot. But right now, the extra burning of oil and coal we’re doing is more than the trees can handle. The amount of carbon in the air is getting ahead of the trees.”

Tom is now coloring his trees green, with brown trunks. “I like thinking about trees and other plants as places to store carbon. Kind of like closets for storing our clothes. And the carbon that is super-stored, way down in the ground—maybe that’s where it should stay? Underground.”

I nod unhappily. “It took many thousands of years to get that carbon down there in the ground. But we’ve burned up most of it in only a few years. Doesn’t that seem kind of unfair to Gaia? When we burn up coal and oil so fast, we’re putting lots more carbon dioxide into the atmosphere, and it’s hard for Gaia’s organs to get rid of it again. So the Earth has been getting warmer. Remember when we found out about greenhouse gases?”

“About that,” says Tom. “I found a video about the carbon cycle that really explains what’s going on.” He goes to the Environmental Protection Agency website and clicks on a video link.

As we watch the video, Julie is unconsciously braiding her hair, something she does when she’s upset or worried. When it ends, she turns to me. “Okay, so we need to stop burning all that coal and oil,” she announces, shaking her head so that the braid comes out. “Another thing is, we could plant more trees to store more carbon.”

“And not cut down so many of the trees we already have,” Tom adds.

“I really liked the part in the video about the bathtub,” says Julie. “It reminded me of the ocean. And I am the Water Woman!” She leaps out of her chair.

I open the refrigerator and take out ham and cheese for the sandwiches we’ll have for lunch. As I spread mustard and mayo on the bread, I ask Julie, “What about the ocean, Water Woman? Are the oceans one way that Gaia keeps the world cool enough to be just right?”

“Oceans are cool, really cool!” Julie answers, looking over at me with an impish grin. “Tomorrow morning I’m going to tell you guys why!”