YESTERDAY AFTERNOON, after Tom told us about gases in the atmosphere, we went for a walk in the woods behind our house. Julie and Tom live in the city, so they don’t get to spend much time in real woods.

Our walk took us along a narrow trail in the cool shade of tall oak and pine trees, with soft pine needles underfoot. As we went along, I told the children how I used to run barefoot along this path when I was their age, pretending to be a young Native American. I would imagine the wild animals watching me as I ran swiftly by—the coyotes and the foxes and the does with their young fawns.

At one point the trail climbs up a hill to a high point that overlooks a pretty pond shaped like a figure eight. We stopped there and sat on the edge of the bluff, gazing out over the water. Two river otters live in that pond, where there’s plenty of fish for them to eat. I described how eels swim up a little stream from the bay into the pond, where the otters like to catch them for supper.

“One time last spring,” I told Tom and Julie, “I saw one of the otters sitting on that little dock, over there at the end of the pond.” I pointed to the dock. “It was holding an eel in its two front paws, like a big, wiggling sausage!”

Heading home along the trail, we checked out the place where the otters cross our path in the winter, as they follow their own trail from the pond to the bay. “You can see their marks in the snow where they climb up out of the pond, then slide down the other side of the hill into a tidal salt marsh,” I explained. “Sometimes in the spring we see them swimming in the salt water, where it runs out of the marsh. It’s a great place for them to find things to eat.”

BEFORE BEDTIME LAST NIGHT, Tom and Julie voted for a pancake breakfast, so I’m up early to make the batter. The kids sleep up in a loft in the room we use as an office. They have to climb a ladder to get to their beds.

I expected them to sleep late, but as I pass the office on my way to the kitchen, I see that Tom is already awake. He’s sitting in front of our computer in his pajamas, looking at stuff on the screen. “Hi, Gramps!” he greets me.

Tom and Julie sleep up in this loft. The ladder is on wheels so they can roll it over to the opening in the loft railing.

“What are you doing up so early?” I ask.

“Reading about carbon dioxide,” he answers. “Did you know that plants breathe carbon dioxide and turn it into to the oxygen we need to live? So awesome!”

I smile. “That’s good news,” I say. “I’m going to mix some flour with milk and sugar to see if it will turn into pancakes. How does that sound?”

Julie pokes her sleepy head over the edge of the loft. “Pancakes!” she says, “Yes! I’ll get dressed and help make them.”

After breakfast we gather again in front of the office computer, so Tom can tell us what he’s been learning about carbon dioxide. I can tell that he’s excited because he’s doing his foot-tapping thing. Julie starts off with a question.

“So, what is the big deal about carbon dioxide? I know it’s a gas, Mister Gas Guy. But what’s it like?”

Tom looks at her for a minute and then asks, “Have you ever had a can of Coke?”

Julie wrinkles her nose. “Duh, of course I’ve had Coke!” She sounds a little annoyed.

“You know all the bubbles in it that sometimes tickle your nose? Those bubbles are carbon dioxide!”

Julie looks a bit surprised. “Really? Is that what makes sodas fizzy? And that’s the same stuff that’s in our atmosphere? Where does it come from?”

Tom’s foot is tapping really fast now. “Us!” he almost shouts. “It comes from us and all the other animals!”

“Say more,” I urge him. “How does that work?”

“Okay, so we breathe in air because we need the oxygen in it, which is another gas.” He’s talking really fast too. “Then the oxygen gets into our bodies through our lungs. And what’s left when we breathe out is carbon dioxide.” In his excitement he jumps out of his chair and starts pacing around the room.

“So where does the oxygen come from in the first place?” Julie asks a little impatiently.

Tom freaks. “Yes!” he shouts. “Yes, Yes, Yes! That’s exactly the right question!”

“Right, then, smarty pants—tell us where it comes from!” Julie wants answers, now.

Tom hops up the loft ladder a couple of steps and throws out his arms, like an actor giving a speech. “Plants! Would you believe it? Plants breathe in carbon dioxide and they breathe out oxygen. It’s amazing!”

He hops down, runs to the window, and points to the woods where we walked the day before. “All those … I thought they were just plain old plants, like trees and grass and stuff. But they’re really oxygen-making machines. They are the reason we are alive!”

“Plants breathe?” Julie looks doubtful. “Like they have lungs or something?”

Tom turns to me. “Gramps, there’s this process called photosynthesis. It’s kind of complicated. Here, let me show you.”

He goes back to the computer, types photosynthesis into the search bar, and clicks on an image.

“Nice,” I say. “That picture is helpful. It might also help to know that photosynthesis is actually two words put together—photo and synthesis. Photo is another word for light. Synthesis means mixing. Light energy from the sun mixes together water from the ground and carbon dioxide from the air. Can you walk us through the process, Tom?”

How Photosynthesis Works
Light energy from the sun mixes carbon dioxide from the air together with water that enters the plant through its roots. The plant gives off the oxygen we breathe.

“I’ll try.” Tom points to the picture. “So here’s the water, coming into the plant through its roots. Water has oxygen in it. And the air all around the plant contains carbon dioxide gas, right?” Julie and I nod, following him so far.

“The sun shines on the leaves of the plant,” Tom plows on. “That energy from the sun makes the water in the leaves mix with the carbon dioxide. When they mix together like that, oxygen comes out of the water and goes into the air. And that’s what we breathe!”

“Well done!” I say. I turn to Julie, who’s sitting on the edge of the table. “Did you get that?”

She hops down and comes close to the computer screen. “I think so.” She traces a path on the picture, going over the steps Tom has just described. But then she frowns. “Wait a minute,” she says, pointing at one part of the picture. “Sugar is coming out too! Is it a sugar plant?”

Tom looks at me again. “I don’t think they mean sugar like we put on our cereal, do they, Gramps?”

“No, it’s not the same,” I agree. “There are lots of different kinds of sugars. The kind that happens in photosynthesis is actually the stuff all plants are made of—whether it’s leaves on a tree or lettuce in our salad. And this kind of sugar is built out of carbon—just like our house is built from wood.”

“I get it!” says Julie, her eyes widening. “Plants breathe in water through their roots and carbon dioxide from the air. The sun’s energy blends them together, and out comes both oxygen and carbon, to help the plant grow. Then we breathe the oxygen and eat the plants! Amazingly cool!”

Tom shakes his head. “No—we don’t eat all the plants,” he points out. “We don’t eat bushes and trees. We just eat the plants people grow on farms. And maybe a few wild plants, like dandelion greens.”

“That’s true,” Julie says. “But other animals eat lots of wild plants. Rabbits eat grass and clover. Cows eat grass too. And the giraffe … it has a long neck, so it can reach way up into trees to eat the leaves.”

I want Tom to keep going with the photosynthesis story. “Tom, you’re the Gas Guy, so here’s another big question. What does photosynthesis have to do with the atmosphere?”

Tom goes back to the picture of photosynthesis on the screen. He looks at it for a minute while he scratches a bug bite on the back of his slightly sunburned neck.

“Gases,” he says finally. “It’s about the two gases. The first one is carbon dioxide. Plants take that gas out of the atmosphere. Which is good, because carbon dioxide traps heat from the sun. We don’t want too much of it in our atmosphere, or we’ll get too hot. So plants are keeping us cool by gobbling up some of the carbon dioxide.”

He stops to form his next point. “The other gas is oxygen. Plants breathe out that gas into the atmosphere. And we need oxygen to keep us alive.”

“Perfecto!” I say happily, and give him a big hug. He looks a little embarrassed but also pleased. “So you’ve explained why plants are so important. They keep us from getting too hot by taking carbon dioxide out of the air. They make the oxygen we need to breathe. And they give us some of the food we need to eat.”

Julie is still standing in front of the computer, gazing at the picture of the plant with carbon dioxide and water going in, and oxygen and plant food coming out. “Wow,” she finally says. “It’s hard to believe how much a little old plant does to keep us alive.”

She thinks for a bit longer, and then says, mostly to herself, “I wonder what happens when plants die?”

Tom has been drawing a picture of a tree with big leaves on a piece of paper. He looks up, surprised at the question. I get up and give Julie a little squeeze. “Don’t worry,” I say. “They leave lots of seeds behind so new plants can grow.”

Then I ask Tom, “Do you know what coal and oil are made of?”

“Not really,” he admits. “Why?”

“Well, it has to do with what happens when plants die,” I explain. “Could you check that out and get back to us? You’re been on the trail of carbon dioxide, and this is part of it.”

Julie looks frustrated. “I’m the Water Woman!” she says. “It’s time for me to tell you guys all about the oceans and water vapor!”

I’m happy that she wants to show her stuff. “Soon, Julie, soon. But first let’s see whether Tom can answer your question about what happens to dead plants.”

She still looks kind of disappointed—until I remind her and Tom about our plan to sail across the bay and have lunch at the ocean beach. “Ready for some beach time?” I ask.

“Yes!” they both shout, and we head out to the kitchen to make sandwiches for the trip.