Twenty Letters

Aunt Reza had spread the seed packets across the kitchen table in rows, and each packet had a strip of paper stapled to it with a line of letters: A's, T's, G's, C's, twenty of them, over and over.

"Why twenty?" Maya asked. She was holding one up to the window light. "Every single one is twenty letters. Not nineteen. Not twenty-one."

"That's the guide," Reza said, not really looking up. She was counting packets under her breath. "Don't mix the rows, please, I will cry."

"Guide to what," said Soren. He had a packet too, turning it over.

"To the spot. The scissors part of the system can't see. It has to be told where to cut. The guide is the address." She waved a hand. "Twenty letters, it finds the matching twenty in the plant, it cuts there. Sixteen, twenty-four, the kit makes them twenty."

"That's not a lot of letters," Maya said.

"It's enough," said Reza, and went back to counting.

Maya put the packet down. "It's not, though. That's what I mean. It's not a lot."

Soren looked at her. He knew the sound of her starting somewhere before she had the why.

"How many letters does a plant have?" he asked Reza.

"In its whole genome? Depends on the plant. Billions."

"Billions," Soren repeated. He put his packet down. "And the guide is twenty."

"Twenty," said Maya. "Out of billions. And it finds the one right spot." She frowned. "How does twenty letters point at one place in billions? That's like giving someone twenty letters and saying, go find the only sentence in the library that starts this way."

"It would have to be the only one," Soren said slowly. "Or it would cut in the wrong place."

"So is it the only one?" Maya turned to her aunt.

Reza stopped counting. "That," she said, "is the actual question. That is the whole job, honestly. You pick a guide that only matches once."

"But how do you know twenty is enough to only match once?"

Reza opened her mouth, then made a face like the answer was longer than she wanted. "Math. Work it out. You've got paper."

Soren already had his notebook open. He wrote a four at the top of the page.

"Four," he said. "Four letters it can be. A, T, G, C."

"Right," said Maya, pulling a chair around. "So one letter, four possibilities. Two letters?"

"Four times four. Sixteen." He wrote it.

"Three letters, four times sixteen. Sixty-four." Maya was leaning over the page now. "It keeps multiplying by four. Every letter you add, four times more possible sentences."

"So twenty letters is four, twenty times." Soren's pencil stopped. "That's going to be huge."

"Do it," Maya said.

He did it. Four, sixteen, sixty-four, two hundred fifty-six. The numbers climbed down the page, doubling and doubling past anything you could feel. He filled the margin. When he got to ten letters he had a number over a million, and he was only halfway.

"We're at ten," he said. "A million."

"Keep going."

He kept going. The number got too long to say. By the time he reached twenty letters the figure ran off the edge of the line and he had to write the rest underneath, a tail of zeros.

"That's a trillion," Soren said. He counted the zeros twice to be sure. "More than a trillion. A thousand billion possible twenty-letter sentences."

Maya went quiet. She was looking at the number, and then at the seed packets, and then back.

"Say it again," she said. "How big is the plant."

"Billions," said Reza, who had stopped pretending to count. She was watching them now.

"Billions," Maya said. "And the number of different twenty-letter sentences is a thousand billion." She looked up. "There are way more possible addresses than there are spots in the whole plant."

"So," Soren said, and stopped, because he wanted to get it right. "So when you pick twenty letters, there are so many ways twenty letters can come out that almost any one you choose only happens once in the whole thing. There aren't enough spots for it to happen twice."

"That's why it's twenty," Maya said. "Not because twenty is a lot of letters. Because twenty letters makes more addresses than there are doors."

Reza was smiling now, the counting forgotten. "Nineteen is usually enough too," she said. "Twenty gives you room to be sure. Go shorter and the address starts showing up in more than one place, and then the scissors cut where you didn't mean."

"That's the scary part," Soren said. "It can't see. It just matches the letters and cuts. So the letters have to be the only ones."

"That's the whole job," Reza said again, quieter this time.

Maya picked up the seed packet again, the one with the twenty letters stapled to it. She read them. A, A, G, C, T, and on, twenty of them.

"This sentence," she said. "This exact one. It's somewhere in here." She held the packet like it weighed more than it had a minute ago. "In the seed. There's a spot, three billion letters of plant, and right at this one place the letters say this, and nowhere else do they say exactly this, all twenty in a row."

"That's how it finds it," Soren said. "It runs along until twenty in a row match. And in three billion letters, twenty in a row only match once."

"Once," Maya said. "In the whole thing."

She held the packet up to the window the way she had at the start, but it was a different thing now, looking through the same paper.

Soren looked at his notebook, at the number with its tail of zeros running off the line, and then at the rows of packets, each with its little stapled address, each address pointing at one place and only one place in a sea of billions.

"There are more ways to write twenty letters," he said, mostly to himself, "than there are places in a living thing to write them."

Reza pushed a stack of packets toward them. "You two want to read me the guides? I'll tell you which plant."

Maya took the top packet and read the twenty letters out loud, one at a time, slowly, the way you read an address to someone who is about to drive a long way to find one door.