Sakura and the Many-Layered Sea: A Tale of Density and Buoyancy

Once upon a time, on the oily shores of the Many-Layered Sea, there lived a girl named Sakura. You might think that an oily sea sounds worrisome, as if it were the result of a spill or disaster, but this was simply how the sea was. The top-most layer was a quarter-mile thick of oil and plentiful with swarms of oil eels. Which were delicious.

Sakura had short, straight black hair, wore little pink shell earrings, and was fascinated by the sea, as she was fascinated by many things. She had so many questions, but she could barely get them out because she was listening so hard, taking in all the information around her.

One night, she and her grandmother were up late repairing fishing nets. Winds moaned around the corners of their little house, and the drumming of the rain on the roof drowned out all but the loudest cracks and pops from the fire.

“The sea dragon is angry with us today,” her grandmother muttered to herself.

Sakura had never seen the dragon who protected the sea, but her grandmother often blamed him when the storms came up.

“Why is the dragon angry with us?” Sakura asked.

“Because we have taken too many of his eels,” her grandmother said. “We try to limit how many we take, but this time of year there isn’t anything else to eat.”

“That doesn’t seem fair,” Sakura said.

“His duty is to protect the sea creatures,” her grandmother said calmly, and for a while they sat in silence, listening to the rain pound like fists against their home.

“Tell me again,” Sakura said, struggling with the unruly fibers of the netting. “Who lives below the oil?”

Her grandmother smiled, the firelight flickering off her glasses, but she didn’t look up from her work. Sakura marveled as her grandmother’s knobby fingers flew deftly across the fibers, untangling and repairing at a blinding pace.

“Below the oil is where the mermaids live, of course.”

“And they will grant you wishes?”

“To a lucky few, yes,” her grandmother said, twisting two strands of rope together. “Or that was how it used to be. They left in my own grandmother’s day. They went to the center of the Many-Layered Sea, where the dragons go to dance, and no one has heard from them since.”

“Why did they go?” Sakura asked.

Her grandmother shook her head. “No one knows.”

“Then how do you know where they went?”

Her grandmother laughed. “I don’t know. That’s a good question. Maybe they told us why they left, and we didn’t listen. Or maybe they didn’t go there after all.”

Sakura considered this, then jumped at a crack of thunder overhead.

*

The next morning, Sakura went out with her father to fish for the oil eels. Rain still poured from the sky in great sheets that sank into the oily waves, descending in great droplets to join the water layer far below. Sakura stared over the side of the boat, her nose close to the shiny surface as the thick waves slowly heaved and ebbed.

No shimmering shadows of eels today. She squinted, trying to make out the water below the oil, straining for a glimpse of a long-nosed mermaid looking back at her, or a flick of tail, but there was nothing. There were other, thicker layers below that, with unnamed armored creatures lumbering through it, she had heard, but she had never seen them either.

Science Note: The Many-Layered Sea has a layer of oil on top and water below that. Why is the oil on top? Oil floats on water because it is less dense. What is density? Well, if you had two buckets, one full of water and one full of oil, the bucket full of water would be heavier. Because the water is heavier, it ends up below the oil.

Density is the amount of stuff a given size object contains. Imagine taking a slice of cake and squishing it into as small of a ball as possible. You’re packing it more tightly, which increases the density!

They caught only a single eel that day, and the next they caught nothing. For the next two months, barely anyone in the village caught anything.

The storms worsened. Great bruised clouds bubbled up, crackling with electricity and sending torrents of rain to be caught by the wind and blown into eyes and faces and open doorways. The whole village huddled together in the great meeting hall.

“We have to stop catching eels,” her grandmother said, speaking in front of a crowd of elders. “We will only anger the sea dragon further.”

“I haven’t had anything to eat in three days,” a young man said. “We have to catch something!”

Sakura raised her hand, but no one noticed her.

“We must be patient,” an elderly man with a long beard said. “If the dragon sees we are obeying his laws, he will send more eels.”

Sakura lifted her hand higher and waved it a bit. The eyes of the elders slid over her, like eels sliding over rocks.

“The sea dragon only cares about sea creatures. He won’t take pity on us,” the hungry young man said.

“Why don’t we ask the mermaids for help?” Sakura cut in, unable to hold back, jumbling her words together until the room filled with a tiny beat of silence.

The silence stretched longer. A few eyebrows lifted. One or two of the elders glanced at Sakura’s grandmother.

Sakura frowned. It was a better idea than doing nothing.

“Thank you, Sakura. Why don’t you go home and tend the fire for me?” her grandmother said.

Sakura left the villagers arguing behind her, but instead of going home, she went to the rocky shore and stood, squinting against the rain, her hair tossed and tangled behind her by the gale. The waves swelled and crashed against the shore, filled with dark shadows of seaweed, but no eels.

There might not be mermaids out there.

The mermaids might already be gone.

She wrapped her arms around her empty belly, shivering.

If they did nothing, the sea dragon might take pity on them and send them enough eels to live. But maybe the sea dragon had decided he didn’t want them eating his eels anymore.

Trying to catch more eels didn’t seem like it would work, either.

Lightning stabbed down from the clouds like an angular trident.

She ground her feet into the smooth pebbles of the shore, listening to the crunch. Then she spun around in a whirl of pebbles, making for the docks.

She unlashed her small wooden boat, tossed the rope in, and jumped in after it. Her hands went through their familiar dance as she heaved up the mast and hoisted her coral-pink sail, webbed like the fingers of a gecko.

Thunder rumbled as she crashed through the swelling waves of oil. It splashed over the sides of her boat, and soon her fingers were slippery with it. The rain beaded up on her skin and trickled across it.

“I’m going to find the mermaids,” she whispered to herself. But what if there weren’t any mermaids, after all?

Science Activity: To see density in action, grab a cup or bowl, a scale, some gravel, and some sand. Fill the cup with gravel and place it on the scale. How much does it weigh? Now, leaving the gravel, pour sand into the cup around it. What happens to the weight? By filling in the empty spaces of the cup, you’ve increased the density! The cup remains the same size, but it’s more tightly packed, so it’s heavier!

*

Sakura piloted her tiny craft up and down the waves for many hours until her stomach heaved along with the waves.

At first, she thought she might be imagining it, but a great rushing sound built and built, like a river heavy with snowmelt cascading over a cliff.

A spiraling whirlpool appeared a few hundred feet from her boat. It whirled faster and faster, and she could see the funnel extending deep into the oil, all the way to the water layer below. At last, in a great spray of water, a fearsome dragon with long whiskers appeared. He bared his golden teeth at her, his eyes flashing like the lightning behind him. Sakura cowered in her tiny boat, crouching behind the mast, and stared up at him wide-eyed. It was the sea dragon himself!

“Go back, Sakura,” the sea dragon said, his voice washing over her like powerful waves that could crush her into the sea floor.

She clasped her hands and bowed, attempting to be as mannerly as she knew her grandmother would recommend. “Hello great sea dragon. It is an honor to meet you. I apologize for disturbing you, but my family is hungry, and I have to do something. I know you don’t want us to eat your eels, so I am going to ask the mermaids for help in finding something else to eat.”

“Your family disobeyed my wishes.” The cold winds whirled around him, lifting his many long whiskers. His long scaly body whipped and curled in the wind like a ribbon.

From her respectful bow, Sakura noticed a tiny creature with large, luminous eyes hovering at the sea dragon’s shoulder.

“I apologize for my ignorance, oh sea dragon, but . . . who is that?” She pointed.

“That is Clyde,” he said in his booming voice.

Clyde waved tentatively.

She waved back, and the sea dragon frowned, his moustache drooping with displeasure.

“He is my assistant.” The sea dragon cleared his throat and resumed his threatening expression. “Go back to your family.” With a crack of thunder, he disappeared.

Sakura couldn’t go back to her family. She already knew the dragon was angry at them. She readjusted her sail and continued on.

The waves rose higher and higher, and she thought she could hear the sea dragon’s growl in the whistling wind.

Lightning struck her mast, and her beautiful sail disintegrated into flames. She quickly patted the flames out with a heavy cloth. She knew better than to toss fire into the Many-Layered Sea.

Her sail was destroyed, so she pulled out her oars and rowed.

The air grew colder and colder. Her breath rose in clouds, and the rain turned to swirls of snowflakes like the cherry blossoms that were her namesake.

The cold made the oil thicker and thicker, and rowing became very difficult, but Sakura pressed on.

“Interesting,” she thought. “The water freezes before the oil does.”

Science Note: Different materials freeze at different temperatures. It’s why we put salt on roads. Saltwater freezes at a colder temperature than water without salt.

At last, the oil froze solid. Sakura sat for a few minutes, straining against the oars, but finally she stood, stepped out onto the frozen surface, and pried her boat up out of the ice. There was a small, boat-shaped indentation in the oil.

“Interesting,” she thought again, despite the cold and her exhaustion. “I wonder what makes the indentation exactly that size.” She’d seen boats laden with goods that were lower in the water. She could imagine that the heavier boats would make a larger indentation.

She put her boat on her back like a turtle shell and trudged up the slippery, frozen surface, shivering. At the crest of a wave, she set her boat down and made a sled, sliding down into the next trough, where she picked it up and trudged up the next hill.

Night fell, and she slept in her boat, shaking with cold and listening to the soft flakes of snow collecting around her.

*

She awoke the next morning in a chilly white drift. The sun glowed out of an empty blue sky.

“At least it’s not snowing anymore,” she thought.

A sound caught her attention. A wet, slapping sound. Her heart leapt. Was that a mermaid?

She clambered over the side of her boat and flopped into the snow on the other side. It was only an eel, stuck partway in the frozen oil. Her stomach rumbled. She was so hungry. She could imagine what it would taste like, and what being full again would feel like. Her hands shook with hunger as she moved towards it.

She stopped a foot away, watching it flail against the snow.

“No,” Sakura thought. “I’m not looking for an eel. I’m looking for mermaids. If we keep eating the eels, we’ll only anger the sea dragon further.”

But she was so hungry. She’d eaten so many eels in her life. Surely one more wouldn’t make too much of a difference.

She bent down, kneeling in the snow with her hands on either side. With frozen fingers, she dug into the solid oil that kept it locked in place.

At last, she gripped the eel carefully and pulled it free, holding it up and examining it.

And then she let it go.

She watched it swish and curl away over the surface. “I’m looking for a new way, not the old way,” Sakura thought. But she wasn’t sure if she would ever find a new way. That might be her last ever meal sliding away from her.

She sighed and pushed herself up, preparing herself to continue carrying her boat across the treacherous landscape.

Her feet sunk into the oil.

Just a few inches, but it was softer. She took a few steps, her feet sinking further and further each time.

She lunged for her boat, but her feet slipped out from under her.

Half swimming, half crawling through the rapidly thawing oil, she thrashed her way forwards, at last gripping the hull of her boat and hoisting herself up over the side. She slipped and skidded as she landed on the bottom of her boat, which rocked with her movements.

The sea was melting.

She nearly cried as the air warmed around her and the familiar lapping of the sea returned.

A warm rush of fire blasted over the top of her boat, blotting out the sky, and Sakura sat up. The air was filled with dragons, darting and curling. Their jeweled sides sparkled in the sunlight like rubies and amethysts, and they danced and twisted around one another.

For many minutes she sat watching them in wonder. This must be the center of the sea. She had made it at last.

*

She peered over the side of her boat, down through the clear oil layer, into the watery depths far below. Was she imagining the shapes darting to and fro? She shouted and waved, but the oil was too thick.

How was she going to get their attention? Prying a piece of metal from her boat she dropped it into the oil. It sank rapidly, shooting through the oil layer. When it hit the water layer, it sank more slowly, but it still sank.

“I wonder why it sinks at different speeds in the different layers,” Sakura muttered to herself, but she didn’t have time to consider this. She had to figure out a way to get the mermaids’ attention.

Science Question: Do you have any guess about why an object would fall more quickly through the oil than through the water?

She looked up briefly, lost in thought, and two inches in front of her face was a tiny little dragon with enormous glowing eyes smiling at her. It waved.

Sakura gasped and pulled back, but quickly recovered herself. “Hello, Clyde!” she said.

“Thank you for not eating me,” he said, briefly turning into an eel and back again.

“That was you?”

“Yes, I heard what you said when you were speaking to my bodyguard earlier. I wanted to see if you truly meant it.”

“Your bodyguard?” Sakura asked. “You mean, you’re the sea dragon?”

Clyde nodded, his large, lamp-like eyes solemn.

“Did you really mean what you said?” he asked. “About finding something else to eat?”

“I did!” Sakura said. “We only want to live, too. And we need to eat something. There’s nothing else for us to eat. Only eels live in the oil.”

Clyde nodded, still eyeing her thoughtfully. “I will give you the tools you need, but I will warn you, sometimes what we seek can blind us to the true solutions to our difficulties.”

Sakura wasn’t sure what to make of this, but she nodded and bowed politely. “Thank you, Clyde. I will do my best.”

With a sound like a wave breaking over rocks, Clyde disappeared. On the floor of the boat were a single glass bottle, a piece of writing paper, an ink pen, and a pile of sea salt.

In her absolute best handwriting, Sakura carefully printed a message, asking the mermaids if they would speak with her.

She put the note in the bottle, sealed it, and placed it reverently into the sea. It floated. Of course, just like her boat.

“Hmmm . . .” She frowned, thinking. “Maybe I need to weight it down a little.”

She uncorked the bottle and poured in a handful of salt.

Recorking it, she placed it again in the oil. It still floated, although this time it was over halfway submerged.

“Hmmm,” she thought. “Interesting. Just like my boat, the heavier I make the bottle, the more of an indentation it makes in the liquid.”

Science Note: Sakura is exactly correct in her observations here. The heavier the boat, the greater the indentation would be. In fact, if you filled that indentation with water, the amount of water you added to the hole would weigh exactly the same as the boat you had just lifted up! This is something called Archimedes’ Principle. You can see this in action in the Phet Density Simulation linked below!

She added a little more salt and tried again. The cork barely bobbed above the surface.

Sakura chewed her lip. “If I make it too heavy, won’t it just sink all the way to the bottom of the sea?” She imagined the other mysterious layers lurking below.

She only had a single bottle. If she made it too heavy, her message would sink right through the water layer, through whatever other layers there were, and down to rest, lonely on the bottom of the sea.

Chewing her lips and scratching her head, she thought carefully. She remembered the piece of metal she’d dropped. It had moved more slowly in the water than in the oil.

“Why does the sea have many layers?” she wondered aloud. “Why is the water below the oil?” She had never considered this before. It is easy to miss how interesting the world around us really is.

The answer came to her in a flash. The water was heavier than the oil was. That was why it was below the oil. The other layers below the water must be even heavier.

She thought about the indentation her boat made in the oil. If her boat were floating in water, would that indentation be smaller? Somehow, she thought so. The heavier the liquid, the easier it would be to float in it, she thought, although she had no proof.

Was there a way she could test this? She only had one bottle. One chance to call the mermaids for help. She imagined her family and her friends back in her village, hungry and arguing over what to do.

She had an idea. Water from the melting snow still sloshed in the bottom of her boat. She pulled off her boots, which were sturdy and built to be oilproof and waterproof. One she filled with water, the other she filled with oil. She then removed her shell earrings and placed one on the surface of the water, the other on the surface of the oil. They floated there like little shell boats.

She watched them closely, but it was hard to tell if one rested lower in the water. So she weighed them each down with an equal amount of salt. Now it was clear. The shell boat floating on the surface of the oil was riding much lower in the water.

Slowly, one grain at a time, she added salt to each boat. They sank lower and lower, and finally she added one more grain to each boat and the shell on the surface of the oil sank to the bottom of her shoe.

The shell on the surface of the water still floated.

Grinning, her heart pounding, she knew she’d figured it out. If she put just the right amount of salt into the bottle, it would sink through the oil but float on the water below. A mermaid was sure to see it then. She took the bottle with her message, added a few more grains of salt, and replaced the cork. Now it just barely floated. She added one more grain of salt and replaced it in the oil.

Science Activity: You can try sending a message to the mermaids, too! Fill a narrow container halfway with water, then pour in some oil (canola oil works) to make a second layer. Next, find a small, sealable container to carry your message. See if you can perfectly weight the container with salt or sand so that when you drop it in, it sinks through the oil but floats on the water.

Her throat clenched as it dropped below the surface of the oil. She watched it going deeper and deeper.

“Please let it stop,” she whispered. If she’d added too much salt, it would sink straight through the water and disappear forever, taking her only chance with it.

It hit the water and slowed, then bobbed back up, floating perfectly at the surface between the two liquids. Sakura almost cried with relief, laughing.

Almost immediately, a watery shadow approached.

Unable to look away or even blink, Sakura gripped the slippery sides of her boat, staring at the shadow.

The oil began to bubble around her, two waves of oil parted, and she found her whole boat sinking into an indentation in the oil that deepened into a valley and then a large pit. Lower and lower she went until the sky was a single blue circle, like a blue sun overhead surrounded by walls of oil. At last, her boat floated on the surface of . . . water. A strange, more stable feeling than floating on oil, she felt. But she didn’t have long to think this, because a mermaid broke the surface and appeared before her, supported by a rising wave of water.

She wore a coral crown and a seaweed tunic. Tiny conch shells adorned her ears and metal rings of various sizes and thicknesses graced her sharp-nailed fingers, which were gripping a barnacle-encrusted trident. Spines sprouted along her shoulders, and when she blinked a second thin film covered her eye briefly from the side, like the double blink of a lizard.

“You dropped this,” she said, handing the bottle to Sakura. Her voice was sharp, and it vibrated and undulated like an out of tune violin.

Sakura gasped and shut her mouth, which had been gaping open as she stared at the mermaid.

“Hello,” she said. “I’m sorry to bother you, but my family is starving.”

The mermaid’s gaze travelled over Sakura’s arms, clothes, and hair. Sakura couldn’t help but notice that the water below was teeming with schools of silver fish, darting and flashing like an enormous, loose-scaled eel.

The mermaid’s eyes widened and welled up with tears. “I’m so sorry to hear that! That’s terrible!”

Sakura’s chest felt lighter. “Thank you so much.” She hadn’t thought further than this point. How did one ask a mermaid for a wish? “Umm . . . could you help us?”

The mermaid looked at her, blinked a few times. “I’m sorry, what?”

“Er, could you help us?”

The mermaid scratched the side of her head with the tip of her trident. “Um, I mean, what did you have in mind?”

“Don’t you . . . don’t you grant wishes?” Sakura asked, blushing.

The mermaid’s face fell, and the tears spilled down her cheeks.

“Oh gosh. I am so, so sorry little split-fin. There was one mermaid who used to pretend she could grant wishes.”

What was a split-fin? She looked down at her legs. She supposed one could think of her legs as a fin divided in half. “What do you mean, pretend?” she asked.

The mermaid looked down at her hands sadly. “Oh, she would wave her trident around, tap their heads with it, give them shiny rocks or clumps of kelp. I . . . I think she liked to think she was helping them.”

Sakura struggled to wrap her mind around this. That couldn’t be true, could it? If mermaids were real, that meant the stories were real! If the mermaids couldn’t help, what had been the purpose of her whole journey?

“Please, there must be something you can do,” Sakura said, her voice taking on a desperate edge. “Can you send fish up into the oil?”

“They would die there,” the mermaid said, looking horror-stricken.

“But, but . . .”

“I’m just a single person like you,” the mermaid said. “I don’t have power over the fish.”

Sakura bit her lip and tried not to cry. She was so hungry. She’d come so far.

“What should I do?” she asked, partly to the mermaid and partly to herself.

“I don’t know,” the mermaid said. “I’m so so so so sorry.” She was crying in earnest now and shaking her head. “Good luck!”

The last of her words became a torrent of bubbles as she sank beneath the waves, and the oil began to refill the hole, lifting Sakura’s boat gently up to the surface.

*

Sakura lay for a long time in the bottom of her boat, staring up into the blue blankness of the sky. She was too weak to move, even if she wanted to. She was alone, with a broken boat, floating in the middle of the sea. She’d gone as far as she could. She’d done what she set out to do, but it hadn’t worked out like she’d thought. If even finding the mermaids didn’t help, what hope could there possibly be?

Then Clyde appeared.

“I assumed if I told you, you would think it was another trick,” he said softly.

Sakura wiped away her tears but didn’t move from the bottom of the boat.

“This whole trip was for nothing,” she said. Every obstacle she’d overcome, every challenge she’d worked through, she had nothing to show for it.

“No journey is ever wasted,” Clyde said.

“I’m hungrier than when I started. My boat is broken. I don’t know how to find my way home. I’m no closer to helping my family.”

“Sometimes,” Clyde said, “we need a reason to take a journey, but we almost always find that what we sought at first wasn’t the right thing to be looking for.”

Sakura’s head lifted an inch and she peered at the cheerful little dragon.

His moustache twirled as he went on. “Would it be irritatingly mysterious if I told you that you’ve already found everything you need to solve your problem?”

“Yes. Absolutely,” Sakura said.

Clyde shrugged. “Well, that’s just my nature, I guess. If it helps, here’s a scone.”

A small yellow lump appeared in her lap.

“What’s a scone?” she said, poking at it. But Clyde had disappeared.

After a few more hesitant pokes, her hunger overcame her, and she took a small nibble. Then she crammed half of it into her mouth, chewed, and swallowed.

Scones, it turned out, were even more delicious than eels.

Sakura inhaled every last crumb of the celestial pastry, her delight overwhelming her urge to grumble about Clyde’s mysterious advice. But after she’d eaten the entire thing and taken a nap in the sunshine while bobbing gently on the waves, she awoke with a start. As she’d slept, her mind had worked, putting together the pieces of what she’d learned on her journey. The weights of the different layers, the plentiful fish far below the surface. The way things floated or sank depending on how much they weighed and how much space they took up.

Science Question: What would you do in Sakura’s situation? Do you have any ideas about how she might help feed her village?

“Okay, Clyde, I see what you mean,” she said to the clouds. Even though her journey hadn’t ended the way she’d expected, she had still found a solution to her troubles. A way to feed her family. She paused. “Still annoying, though!”

Now, how to get home? She rubbed her hands together and picked up her oars.

On her way home, she briefly invented two different ways of navigating. One with the stars, and one with the sun and the currents.

When she arrived at her village, everyone looked about as happy as she’d felt before Clyde had given her the scone. Unfortunately, she didn’t have any more scones to distribute.

Her grandmother ran down the rocky beach and helped Sakura pull the boat up the shore. She hauled her out and wrapped her in an enormous hug while berating her for running off alone like that.

When Sakura was finally able to detach herself, she called the rest of the villagers to her. She took one of their eel traps, tied a rope to it, and then lashed it to the perfectly weighted bottle, which the mermaid had given back to her. Then she dropped it off the end of their longest dock.

The villagers all peered over her shoulder in confusion, giving each other worried looks as the trap dropped down through the oil and into the water below, where it bobbed calmly. Sakura waited a few minutes, then hauled it back up. It was absolutely bursting with fish.

The villagers stared at her in disbelief. Moments ago, they’d been ready to give her grandmother their condolences that Sakura had gone mad. First there was silence and confusion, and then cheers erupted.

Sakura was hugged and then hugged again. The villagers lifted her into the air, dancing with her on their shoulders all the way back to the meeting hall where people made short work of cooking up the fish. The feast lasted a full week, with more dancing and singing and rejoicing, and Sakura was asked to demonstrate her invention over and over again. She told her story again and again, and her friends and family laughed and cried in all the right places.

One person suggested that the mermaid really had helped. After all, she’d given Sakura back the bottle. Sakura didn’t agree, didn’t want to perpetuate the myth that mermaids could grant wishes. On the other hand, if Sakura hadn’t believed there was a magical solution to her problem, she might not have looked for a solution at all. She eventually concluded that it was complicated and stopped correcting people.

From then on, the villagers only took a few eels from the sea and instead fished in the deeper layers. Eventually, the layers below the water were explored, too, and even more wonderous creatures and plants were discovered there. The people learned to be better stewards of all the layers, and everyone lived in harmony and abundance.

As for Sakura, she made many more inventions and discovered many interesting things about the world. She also spent many years discovering and then perfecting a recipe for scones. Occasionally, Clyde would visit and give her cryptic advice for his own amusement, which Sakura often decoded and used to her benefit. And they all lived happily ever after.

The End

This story comes from my book, Physics Fables.