"Wonderful!" said Squerib's mom, when she saw her kid's quiz of multiplication facts. "A perfect score!"
Math, it had always been Squerib's best subject. Reading ze'd struggled with, science was sort of okay, but ze was exceedingly good at math. Even last year, in third grade, when the rest of zir class was doing subtraction, ze could have done perfectly on this quiz. It was like ze had a calculator instead of a brain.
"Yes. Again. Tomorrow we start talking about prime and composite numbers," ze said. "I'll probably be bored. Again."
Ze ate dinner, studied for history, and then went to bed.
The next day, as predicted, the teacher talked about prime numbers. "A prime number", she said, "is a number that you can't get by multiplying two numbers together. For example, the number 7 is prime, whereas the number 12..."
"...is not prime, because it's 3 times 4", muttered Squerib to zemself, in unison with the teacher. Ze looked at zir digital watch. It said that it was 12:33. 1233 = 9 × 137, ze thought to zemself.
"You can tell if a number is prime by trying to divide it by every number below it."
The student sitting in the desk next to Squerib raised zir hand. "What if you have a really big number, like 2,361,426,689?" ze asked.
"Then it'll take you—"
Squerib interrupted: "It's 39409 times 59921."
"Don't interrupt", said the teacher. "It'll take you a really long time."
Squerib stopped paying attention to the teacher and noticed that the student who had asked the question was using zir calculator.
"Did you do that in your head?" the student whispered to Squerib.
"Yeah", said Squerib.
"How?" ze asked.
"I don't know. It just comes naturally to me."
A few days later, the word had started to get around that Squerib had really good ability at prime recognition.
"Is 239 prime?" a student might ask.
"Is this for homework?" asked Squerib.
"No, not at all", said the student.
"It is prime", said Squerib, and continued on zir way to class while the other student looked in zir backpack for zir math homework.
When class got out, ze went in front of the school to wait for zir mom like ze usually did. While ze was waiting, an adult came up to zem who ze had never seen before.
"So you're good at recognizing prime numbers?" asked the stranger.
"Uh..." Squerib started to say. Ze wasn't really comfortable talking to people ze didn't know.
"How about this number?" said the stranger, and he gave Squerib a piece of paper with a string of over a hundred digits on it.
"Why are some of these letters?" asked Squerib.
"Because it's hexadecimal. Instead of the value of each digit being ten times the next, each one is sixteen times, and the letters A through F stand for the digits ten through fifteen."
"Oh, I see. Is this some crazy adult math homework?" asked Squerib.
"No, not at all", said the stranger.
Squerib stared at the number for a second. While ze wasn't normally okay with talking to strangers, ze didn't see the harm in telling them something like this. "It's composite."
"And the factors?"
Squerib told him the factors. The stranger wrote something down on his paper and left.
Soon after, Squerib's mother came, and Squerib got into the car.
On the way home, Squerib asked zir mom, "I'm curious, what are prime numbers used for?"
"Well, the main use that I know of is cryptography", said zir mom. "If two sufficiently large prime numbers are multiplied together and someone only has the result, it takes thousands of years to figure out what the factors are. I don't know exactly how it works, but it's used in things like storing passwords and credit card numbers."
"But it can't be that hard to figure out the factors; I can do it in my head!" said Squerib.
"The numbers they use for cryptography are way, way, way bigger than the ones you're dealing with in class. I doubt you could do it with those", said zir mom.
Squerib didn't know what to say, so ze remained quiet.
The next morning Squerib came downstairs as usual to have breakfast. While ze didn't normally read the paper, a particular headline on the front page caught zir eye: "Hacker Steals Millions of Credit Card Numbers". While zir mother was in the kitchen, ze looked at the article. While there wasn't much that ze found interesting in the article, it did say that they'd traced the IP address of the hacker to somewhere in the town ze lived in.
All day at school, ze worried about what this might mean. Did ze unintentionally give the hacker what he needed to get those credit card numbers? Finally school was over and ze could go home.
"M–mom?" ze said, tentatively, after ze'd gotten home. "You know that article... in the paper... about the hacker?"
"I thought you didn't read the paper", said zir mom. "But yeah, what about it?"
"I... might have... um... accidentally helped that person."
"How?"
"There was a stranger outside the school yesterday while I was waiting for you. He asked me to factor a number for him."
"What kind of number?"
"Long. Like, over a hundred digits, I don't remember exactly. And it had letters in it, he said it was in hexadecimal."
"Hmm... what did this person look like?"
Squerib gave a description of the stranger.
"Hold on", said zir mom, and went into the next room to use the phone. When she came back, she said, "The police want to see you at the police station tomorrow after school."
"Am I in trouble?" asked Squerib.
"I hope not", said zir mom.
Squerib was nervous as ze walked into the police station with zir mom. Zir mom introduced Squerib and herself to the police officer.
"I was expecting you to be a bit older", said the police officer. "However, congratulations. We've caught the person responsible for stealing the credit card numbers, and it's mostly thanks to you!"
So in the end it all worked out, but ze'd learned an important lesson: Don't talk to strangers. Otherwise you risk telling them something they weren't smart enough to figure out themselves, and they could use that information for harm.
After that they went home, Squerib did zir homework, and then ze and zir family went to zir favorite restaurant.