Sam carried the robot into school under a bedsheet, which made it look mysterious and important and also kept the wind from catching the cardboard wings. The robot was not a secret, exactly. But Sam wanted it to make it to the gymnasium without anyone seeing it first, because once people saw it, they would ask questions, and Sam was not sure yet what the answers were.

The gymnasium smelled like floor wax and old projects from years past. Sam's classmates had already set up their booths. There was a volcano that erupted with red vinegar foam. There was a model of the solar system with a working light-up sun. There was a poster about recycling that had won a prize at the county fair last year, which seemed like cheating but also seemed like exactly what posters were supposed to do.

And then there was Mia's robot.

Mia's robot was in a clear plastic case and came with a remote control. It could walk forward and backward, lift small objects, speak five languages, and solve a Rubik's cube in under two minutes. It had cost three hundred and forty-seven dollars, which Mia mentioned once and then did not mention again, because Mia was not actually trying to brag. She was just reporting facts. The robot was impressive, and everyone knew it, and Sam knew it too, standing at the end of the aisle with a bedsheet over something that looked, from certain angles, like a pile of recycling that had been arranged with hope.

Sam had built the robot from a cardboard box, eight bottle caps, two motors out of an old electric toothbrush, a speaker from a broken music box, and a bunch of wires that Sam had learned to solder by watching a video on a phone borrowed from Dad. It had taken a week. It had fallen apart twice. The first time it fell apart, Sam sat on the basement floor and thought seriously about becoming a professional video game player instead. The second time it fell apart, Sam just put it back together, because by then Sam knew how it went together, which was the whole point.

Mr. Patterson, who taught science and also sponsored the science fair, walked up and down the aisles checking everyone's set-up. He stopped at Mia's robot and said "Wow" in a way that was genuine and not performative, because Mr. Patterson was like that. He asked Mia how long she'd been working on it and she said "I didn't build it, my parents got it for me," and Mr. Patterson nodded and said "That's great that you chose it for your project" and wrote something on his clipboard.

He came to Sam's table. Sam had uncovered the robot by then. It looked better uncovered, mostly. The body was a large cardboard box with the flaps folded down and taped, which Sam had thought was a good design until now. The arms were the legs of an old pair of reading glasses, bent at the elbow joints and attached with twist ties. The face was two bottle caps, which Sam had painted silver, with a smile drawn underneath in marker. On its chest were six more bottle caps, arranged in two rows of three, like buttons on a spaceship control panel. They did not do anything. They were decorative. Sam was not sorry about this.

"What does it do, Sam?" Mr. Patterson asked.

Sam looked at the robot. The robot looked at Mr. Patterson with its bottle cap eyes.

"It can move forward," Sam said. "And it can turn. And it can tell when something is in front of it, using the sensor from the toothbrush."

"That's actually really cool," Mr. Patterson said. "The obstacle detection — that's a real engineering challenge. Can you show me?"

Sam pressed the button on the robot's back — the button Sam had spent three hours figuring out, because the robot needed a way to turn everything on without taking apart the battery pack every time. When Sam pressed the button, the robot made a sound. A small sound, from the speaker, like a musical phrase played on an instrument that was not quite tuned. And then the robot moved forward, slightly, and stopped when it detected Mr. Patterson's shoe.

"Good, good," Mr. Patterson said. "Sensors are working."

Sam pressed the button again. The robot made the sound again. This time, instead of stopping at Mr. Patterson's shoe, it turned in a small circle, and then from somewhere inside its cardboard body, a voice emerged — not the musical phrase but something Sam had recorded, one take, with the phone borrowed from Dad:

"You did this yourself."

Mr. Patterson looked at the robot. The robot looked at Mr. Patterson. Sam looked at both of them, waiting for whatever came next.

"I'm sorry?" Mr. Patterson said.

"It's what it says," Sam said. "When it finishes doing something."

Mr. Patterson was quiet for a moment. Then he smiled, not the big performative smile he gave the volcano but a smaller one, the kind that meant he had noticed something that mattered.

"You built this entirely yourself?" he asked.

"The cardboard was from a appliance box," Sam said. "The bottle caps were from the recycling bin. The motors were from a toothbrush. But yeah. I built it."

Mr. Patterson wrote something on his clipboard that was not the same something he'd written at Mia's table. He moved on.

Marcus came over at lunch. Marcus was Sam's best friend and also the most honest person Sam knew, which was a combination that could be difficult. Marcus looked at the robot for a long time.

"It doesn't do very much," Marcus said.

"I know," Sam said.

"Mia's robot can solve a Rubik's cube."

"I know."

"Can yours do anything hers can't?"

Sam thought about this. "It can tell you that you did it yourself," Sam said. "Hers can't do that."

Marcus thought about this too. He looked at the robot, and then he looked at Sam, and then back at the robot.

"Can I press the button?" he asked.

Sam nodded.

Marcus pressed the button. The robot played its little musical phrase, moved forward, detected Marcus's shoe, stopped, and then said in Sam's recorded voice: "You did this yourself."

Marcus laughed. Not a polite laugh. A real one, the kind that meant he had been surprised by something being exactly what it should be.

"Okay," Marcus said. "That is actually kind of the best thing I've ever heard."

More kids came over during the afternoon. Some of them compared Sam's robot to Mia's robot, which was natural and inevitable and also uncomfortable in ways Sam didn't want to think about too hard. But Mr. Patterson had said the obstacle detection was cool, and Marcus had laughed, and Sam's mom, who had come to help pack up at the end, had gotten very quiet when the robot said "You did this yourself" and then looked at Sam instead of at the robot.

"Baby," Sam's mom had said, which was her way of saying something she didn't have words for yet. She had squeezed Sam's shoulder and not said anything else, which was exactly right.

At the end of the fair, Mr. Patterson gave out the awards. Mia's robot won for Best Engineering, which everyone agreed was correct and fair. A project about mold growing on bread won for Best Hypothesis, which Sam thought was secretly the most interesting one. The robot made from recyclables did not win anything.

Sam walked home with Mom in the late afternoon light, carrying the robot under the bedsheet again because it was too good to be left in the gymnasium overnight. The robot was lopsided — one arm was slightly higher than the other because Sam had repaired it wrong the second time it fell apart and run out of twist ties.

"Are you sad?" Mom asked.

Sam thought about it honestly, the way Marcus would have.

"A little," Sam said. "But also — I don't know. The robot does the thing it's supposed to do."

"What thing?"

"It reminds me," Sam said. "That I did it."

Mom didn't say anything for a block. Then she said, "You know what's funny? I think that's the only thing any of us are really trying to build."

"What do you mean?"

"Nothing," Mom said. "You're right. It does the thing it's supposed to do."

They walked home. Sam put the robot on the shelf next to the bed, where it could see the room but couldn't be stepped on accidentally. The bottle caps caught the light from the window and turned silver and gold. It looked like what it was: a thing that someone had made, with their hands, from other things, because they wanted to see if they could. And then, because they could, they had.

Sometimes, late at night, Sam would press the button just to hear it say it. Not because Sam needed reminding. But because it was nice to hear, and because it was true, and because some things are worth saying out loud, even when no one else is listening.