How Do Fish Breathe?

Mabel pressed her nose right up against the glass.

On the other side, a little orange fish pressed its nose right up against the glass too.

They stared at each other.

"Excuse me," Mabel whispered to the fish. "How do you breathe in there?"

The fish opened its mouth. Then it closed its mouth. Then it opened its mouth again.

"That's not really an answer," said Mabel.

It was a Tuesday, which meant it was Aquarium Day. Mabel's class had arrived that morning in a big yellow bus, and everyone had been given a partner and a worksheet and a pencil that was too short. They were supposed to be drawing their favorite sea creature.

But Mabel could not draw her favorite sea creature because she was too busy thinking.

She watched the little orange fish swim in a slow circle. It didn't have a nose — well, not a real nose, not like hers. And it definitely didn't have any lungs. She'd learned about lungs last month. You breathe air in, and your lungs puff up like balloons, and that's how oxygen gets into your body. Easy.

But this fish was surrounded by water. Only water. No air anywhere.

"Aren't you worried?" Mabel asked the fish.

The fish did a little flip and swam behind a rock.

"Mabel!" called her teacher, Ms. Reilly. "Please stay with your partner!"

Mabel's partner was a boy named Oliver, and Oliver was already three rooms ahead, because Oliver walked extremely fast and never looked at anything for more than two seconds. Mabel walked extremely slowly and looked at everything for longer than two minutes. They were not a natural pair.

Mabel found Oliver in the jellyfish room, where the jellies floated in purple light like tiny glowing ghosts.

"Oliver," she said. "How do fish breathe?"

"With gills," said Oliver, already walking away.

"But HOW?" said Mabel.

Oliver was gone.

Gills. Mabel wrote the word on her worksheet. She underlined it. She put a question mark next to it. She put three more question marks next to that.

The jellyfish pulsed and drifted. Mabel watched them. Did jellyfish have gills? Did jellyfish breathe at all? She shook her head. One mystery at a time.

She walked into the next room, which was the big one — the one with the enormous tank that stretched from floor to ceiling. Inside it, a sea turtle glided past, slow and graceful, like it had absolutely nowhere to be and was perfectly happy about it.

Mabel sat down on the bench in front of the tank. She hugged her knees.

"Okay," she said quietly. "Let me think about this."

She knew that people breathe oxygen. She knew that oxygen is in the air. But wait — her dad had told her once that there's oxygen in water too. Water is made of hydrogen and oxygen, all mixed together.

So the oxygen was right there in the water, all around the fish, all the time.

"But how does the fish GET it out?" Mabel murmured.

A stingray swooshed past the glass, flat as a pancake.

Mabel thought about her own breathing. Air goes in through her nose, down to her lungs, and her lungs somehow grab the oxygen and put it in her blood. Her lungs were like tiny oxygen catchers. Like a net for the smallest, most invisible butterfly in the world.

So maybe gills were like that. But for water.

She stood up and walked very close to the glass again. A silver fish with a grumpy face swam by, and Mabel watched carefully. She could see thin slits on the sides of its head, just behind its eyes. As the fish swam, the slits opened and closed, opened and closed, in a gentle rhythm.

"THAT'S them!" Mabel said, way too loud. A man on the bench behind her jumped and spilled his coffee.

"Sorry," said Mabel. "But look — those slits! Those are the gills!"

The man did not look. He was busy wiping coffee off his pants.

But Mabel didn't notice. She was watching the fish open its mouth, and she could see it now — the fish was gulping water in through its mouth, and the water was flowing over the gills, and the gills were catching the oxygen, and then the water flowed back out through the slits.

It was like breathing, but with water instead of air.

"You're BRILLIANT," Mabel told the grumpy silver fish.

The grumpy silver fish did not seem flattered.

Mabel grabbed her worksheet and started drawing. Not the drawing she was supposed to do — a different drawing. She drew a fish with big arrows showing water going in the mouth and out the gills. She wrote "OXYGEN" in wobbly capital letters with an arrow pointing to the gills. She drew little sparkle marks around the word because oxygen felt like something that deserved sparkles.

"Mabel!" It was Oliver. He had circled the entire aquarium and found her again. "Ms. Reilly says we have to go to the touch tank now."

"Oliver, look!" Mabel held up her drawing. "The fish takes the water in through its mouth, and it flows over the gills, and the gills grab the oxygen out, and then the water goes out the side of its head! It's like underwater breathing! The gills do what our lungs do, but with WATER!"

Oliver looked at the drawing. He tilted his head.

"That's actually pretty cool," he said.

"I KNOW," said Mabel.

They walked together to the touch tank, and for once, Oliver didn't walk too fast. He was asking Mabel about the gills, and Mabel was explaining, and she used her hands a lot because some things are better explained with hands.

At the touch tank, there were little sharks — nurse sharks, the gentle kind. Mabel reached in slowly and touched one on its back. It felt like wet sandpaper. And right there, on the side of its head, she could see the gill slits. Five of them, in a row, like tiny window blinds.

"Oh," Mabel breathed. "You have them too."

The little shark glided forward, and Mabel felt it slip under her fingers like a secret.

"Ms. Reilly," Mabel said. "Did you know sharks have FIVE gill slits, but most fish only have one on each side?"

Ms. Reilly smiled. "I did not know that, Mabel. Where did you learn it?"

Mabel pointed at the shark. "From her."

On the bus ride home, Mabel sat next to Oliver. She looked out the window at the sky, the big wide-open sky full of air, and she took a deep breath. She felt her lungs fill up. She felt her chest rise.

Somewhere, back at the aquarium, a little orange fish was opening its mouth, letting water rush over its gills, pulling oxygen out of the sea.

Same thing, Mabel thought. We're doing the same thing. Just in different worlds.

She pressed her nose against the bus window, the way she'd pressed it against the aquarium glass, and she watched the world go by — full of invisible, sparkle-worthy oxygen.

And she breathed it in.