What Makes Thunder?

Darcy pressed her nose against the window and watched the sky turn the color of a bruise. The trees in the backyard leaned sideways, and the first fat drops of rain smacked against the glass like tiny water balloons.

Then came the thunder.

BOOM.

It rolled across the sky like someone had knocked over every pot and pan in the world's biggest kitchen.

Darcy's little brother, Miles, grabbed the couch cushion and pulled it over his head. "I don't like it," he said, his voice muffled by the pillow.

"I'm not scared," Darcy said. And she wasn't. She was something else entirely. She was curious.

"Mom," Darcy called into the kitchen. "What makes thunder?"

Her mom was stirring something on the stove that smelled like tomatoes and garlic. She smiled and said, "Oh, that's just the clouds bumping into each other, sweetie."

Darcy thought about this. She squinted. Clouds were fluffy. She'd seen them a million times. Fluffy things don't make that kind of noise when they bump together. She had bumped two pillows together just last week during a pillow fight with Miles, and the sound was more of a fwump than a BOOM.

"Are you sure?" Darcy asked.

"Mm-hmm," her mom said, tasting the sauce. "That's what my grandmother told me."

Darcy loved her mom. But she wanted the actual answer.

She found her dad in the garage, organizing his toolbox. He was the kind of person who liked to sort screwdrivers by size, which Darcy respected.

"Dad, what makes thunder?"

Her dad set down a wrench and rubbed his chin. "Well, Darcy-girl, the way I heard it, thunder is the sound of angels bowling."

"Bowling," Darcy repeated.

"Yep. Big ol' bowling alley up in heaven. When you hear the thunder roll, that's someone going for a strike."

CRACK-BOOM!

Another one shook the house. The lights flickered.

"That was definitely a strike," her dad said with a wink.

Darcy loved her dad. She loved that he winked and that he sorted his screwdrivers. But angels bowling was not the actual answer, and she knew it.

She told her mom she was going next door to Mr. Peña's house, and then she ran — because the rain was really coming down now — and knocked on his door. Mr. Peña was retired and knew things about a lot of stuff because he'd been a science teacher for thirty-one years. He had a garden full of tomatoes the size of softballs, and he always said hello to Darcy's cat, Gerald, by name.

Mr. Peña opened the door. "Darcy! You're soaking wet."

"Mr. Peña, what makes thunder?"

Mr. Peña's eyebrows went up. Then he smiled — not the kind of smile adults give you when they think your question is cute, but the kind of smile that meant he thought it was a great question.

"Come in," he said. "Let me get you a towel. And a cookie. This is a cookie-worthy question."

Darcy sat at Mr. Peña's kitchen table with a towel around her shoulders and an oatmeal raisin cookie in her hand. She would have preferred chocolate chip, but the answer mattered more than the cookie.

"Okay," Mr. Peña said, sitting across from her. "First, tell me what you already know about lightning."

"It's the bright part," Darcy said. "The zippy flash."

"Right. Lightning is a giant spark of electricity. You know how sometimes in winter, you shuffle across the carpet in your socks and then touch a doorknob and — ZAP?"

"Yes!" Darcy said. "Miles cries every time."

"Well, lightning is like that, but enormous. A huge zap between the clouds and the ground — or sometimes between one cloud and another cloud. And here's the thing, Darcy. That bolt of lightning is unbelievably hot. Hotter than the surface of the sun."

Darcy's eyes went wide. "Hotter than the sun?"

"The surface of the sun, yes. And when something that hot moves through the air that fast, it heats up the air around it so quickly that the air expands — it blows outward like a balloon popping, but way bigger. That rushing, exploding air creates a sound wave." He clapped his hands together, hard. CLAP! "That sound wave is thunder."

Darcy sat very still. She was thinking.

"So thunder is the sound of air getting out of the way?" she said.

Mr. Peña pointed at her. "That is a wonderful way to put it. Yes. Lightning is so hot and so fast that the air basically shouts."

"The air shouts," Darcy repeated. She loved that.

"And you know why you see lightning before you hear thunder?" Mr. Peña asked.

Darcy shook her head.

"Because light travels faster than sound. The flash reaches your eyes almost instantly, but the sound takes its sweet time strolling over to your ears. That's why sometimes there's a gap. Flash… then boom."

Right on cue, the kitchen lit up white. One-Mississippi. Two-Mississippi. Three-Miss—

BOOM.

Darcy grinned so big that oatmeal raisin crumbs fell off her lip.

"So if I count the seconds between the flash and the boom, I can tell how far away the lightning is?" she asked.

Mr. Peña leaned back in his chair. "Roughly five seconds per mile."

"That one was about three seconds. So it's less than a mile away?"

"Now you're a scientist," Mr. Peña said.

Darcy finished her cookie, thanked Mr. Peña three times — once for the towel, once for the cookie, and once for the actual answer — and ran back home through the rain.

She burst through the front door, dripping all over the hallway rug. Miles was still on the couch, still under the cushion.

"Miles," Darcy said, sitting down next to him. "Do you want to know what makes thunder?"

A small, suspicious eye appeared from under the cushion. "What?"

"Okay. So you know lightning? The bright zippy part?"

"I don't like the bright zippy part," Miles said.

"I know. But listen. Lightning is a giant spark, like when you touch the doorknob in your socks. And it's SO hot — hotter than the sun —"

"Nuh-uh," Miles said.

"Yes-huh. And it's so hot that the air around it goes WHOOSH and blows outward really fast, and that is what makes the booming sound. Thunder is the sound of air shouting because lightning surprised it."

Miles thought about this. He lowered the cushion a little bit. "The air gets surprised?"

"Wouldn't you be surprised if something hotter than the sun zapped right through you?"

"...Yeah."

FLASH.

Darcy grabbed Miles's hand. "Count with me. One-Mississippi, two-Mississippi, three-Mississippi, four-Mississippi, five-Miss—"

BOOM.

"About one mile away!" Darcy said.

"Wait — it moved farther?" Miles said, sitting up now.

"Yes! The storm is moving away from us."

They waited. They watched.

FLASH.

"One-Mississippi, two-Mississippi, three-Mississippi, four-Mississippi, five-Mississippi, six-Mississippi, seven—"

BOOM.

"It's leaving!" Miles said. He was out from under the cushion completely now. He was kneeling on the couch, nose against the window, right next to Darcy.

They counted together four more times. Each time, the gap got longer. Each time, the thunder got softer. And each time, Miles smiled a little more.

Their mom came in from the kitchen, drying her hands on a dish towel. She looked at both of them pressed against the glass and said, "What are you two doing?"

"Measuring the storm," Darcy said.

"With Mississippis," Miles added.

Their mom tilted her head. "I thought it was clouds bumping together."

Darcy looked at her mom. She loved her very much.

"Not exactly," Darcy said. And then she told her.