Chapter 1: The Rumble Before the Exam: When Your Gut Talks Back

The shriek of Mia’s cracked iPhone alarm, blaring the latest TikTok viral sound at top volume, yanked her out of a half-dream where she was floating in a beaker of fizzing green chemistry solution. She slapped the snooze button so hard the screen cracked a little more in the corner—she’d been meaning to get a new case for three weeks, but between AP chem homework, soccer practice, and scrolling Reels until 2 a.m., she’d never gotten around to it. The second the alarm cut out, a low, gurgling cramp twisted in her lower abdomen so sharp she gasped, curling into a ball on her threadbare cartoon cat bedsheets. Her backpack, slung over her desk chair the night before, was still stuffed with her half-folded chem review sheet, and her Genshin Impact keychain glinted in the dim morning light.

This wasn’t new. Every big test since freshman year had started this way: the gnawing stomach cramps, the brain fog that made even simple equation feel like hieroglyphics, the urgent, panicked need to sprint to the bathroom five minutes before the exam started. She’d written it off as test anxiety for months—her mom had the same thing before big work presentations, her older cousin got so nervous before his SATs he threw up in the car on the way to the test center. But this time felt worse. Last week’s chem midterm practice test had earned her a 72, and Mr. Henderson had circled her score in red marker, writing *“Needs serious review before Friday’s exam”* in the margin. She’d spent the past three nights cramming until 1 a.m., surviving on Monster energy drinks, takeout dumplings, and the half-eaten cheeseburger she’d shoved in her backpack after soccer practice yesterday.

She rolled out of bed, her legs shaking a little, and yanked open her closet. The hoodie she’d worn to soccer practice still smelled like grass and sweat, but it was the only clean one she had. She pulled it on, grabbed her backpack, and padded downstairs, the wooden steps creaking under her feet. The smell of frying bacon hit her as soon as she stepped into the kitchen, and her stomach roiled so violently she had to lean against the doorframe. Her mom was at the stove, flipping a piece of extra-crispy bacon, and her little brother was sitting at the table drawing dinosaurs on his placemat with crayon.

“Morning, sleepyhead,” her mom said, not looking up from the pan. “I made pancakes, thought you’d want something filling before the test.”

Mia shook her head, her mouth already watering with the urge to be sick. “No thanks, I’m not hungry. Can I have a banana? I need to run, the bus will be here in five minutes.”

Her mom frowned, turning to look at her. “You look pale. Are you okay? You’ve been complaining about your stomach all week.”

“I’m fine,” Mia said, forcing a smile. She grabbed a banana from the fruit bowl, her hand trembling a little. “Just test anxiety. Happens every time. I’ll be fine once the test starts.”

She was halfway out the door when her mom called after her: “If it gets worse, call me, okay? I’ll come pick you up.”

Mia waved a hand over her shoulder, already jogging down the front walk. The cool October air bit at her cheeks, but she was sweating under her hoodie. She could hear the school bus’s brakes squealing two blocks away, and she picked up speed, her backpack bouncing against her back. Halfway to the bus stop, the cramp in her stomach twisted so sharp she stumbled, dropping to her knees on the sidewalk. A neighbor walking their golden retriever glanced at her, concerned, but Mia waved them off, breathing hard through her nose. By the time she got to the bus stop, the bus was already pulling away, its yellow side flashing in the morning sun.

She stood on the corner for ten minutes, trying to flag down a passing car, before an Uber pulled over. The driver had country music playing low on the radio, and the smell of his pine tree air freshener made her stomach roll so badly she had to roll down the window and stick her head out for half the ride. By the time she got to Westbrook High, she was ten minutes late to first period, and the office gave her a pass to go straight to Mr. Carter’s biology class, which was her second period of the day.

She slipped through the classroom door as quietly as she could, but the old wooden floor creaked under her sneakers, and the whole class turned to look at her. Mr. Carter was standing at the front of the room, a video playing on the smart board, and he smiled when he saw her, holding up a hand to silence the giggling that had started to bubble up from the back row.

“No problem, Mia,” he said, gesturing to an empty seat next to Lily, her best friend. “We were just getting started. Grab a seat.”

Mia sank into the chair next to Lily, her face burning. As soon as she sat down, her stomach let out a low, rumbling gurgle that echoed through the quiet classroom. A kid in the back row snickered, and Mia buried her face in her arms on the desk, wishing she could disappear. She’d had a bad feeling about this morning the second she woke up, and now she was sure the whole class thought she was a weirdo who couldn’t control her own body.

“Rough morning?” Lily whispered, passing her a note that had *Did you study the electron transport chain?* written on the front.

Mia nodded, scribbling back *Stomach is killing me. Think I’m gonna fail*. She was about to write more when Mr. Carter’s voice cut through the quiet.

“Before we dive into today’s lesson, I want to ask a question,” he said, leaning against the front of the desk. “How many of you have ever gotten really nervous before a test, and had your stomach hurt so bad you thought you were gonna throw up? Or had to run to the bathroom right before an exam started?”

Half the class raised their hands. Mia’s hand shot up so fast she knocked her pen off the desk.

“I thought so,” Mr. Carter said, smiling. “Most of you probably think that’s just ‘test anxiety’—your brain being scared of the test, making your stomach upset. But what if I told you that’s not the whole story? What if I told you that the ‘anxiety’ you feel in your stomach is actually coming from the trillions of tiny living things that live inside your digestive tract, talking to your brain?”

The classroom went quiet for a second, then a kid in the back row called out, “Wait, what? There are living things in my stomach?”

Mr. Carter laughed. “Oh, there’s way more than just ‘living things.’ Let me show you.”

He clicked the remote, and the video on the smart board changed. It was a bright, cartoon animation of a tiny spaceship flying through a winding, glowing tunnel, surrounded by millions of tiny, colorful blobs of different shapes and sizes. A cheerful voiceover started talking: *“Welcome to your gut! This tunnel is your digestive tract, and all these tiny blobs are your gut microbiome—trillions of bacteria, viruses, fungi, and other microbes that live inside you, mostly in your large intestine. There are more of them than there are people on Earth, and they’re not just freeloaders. They’re your allies, your helpers, your second brain.”*

Mia stared at the screen, her stomach cramps fading a little as she got interested. She’d heard of “gut bacteria” before—her mom was always talking about probiotics and yogurt—but she’d always thought it was just another weird adult health fad, like kale smoothies or cryotherapy. She’d never thought those tiny bugs had anything to do with her test anxiety.

“Let’s break this down,” Mr. Carter said, pausing the video. “First, let’s talk numbers. The average human body has about 30 trillion human cells. But your gut microbiome has around 38 trillion microbial cells. That’s more cells than you have in your entire body, and they weigh about 2 to 3 kilograms total—roughly the same weight as your brain.”

A kid in the front row whistled. “That’s heavier than my cat.”

“Exactly,” Mr. Carter said. “And these microbes aren’t just hanging out. They have over 3 million genes total, compared to the roughly