The scab on Soren's forearm had started to itch, which his mother said meant it was healing. He didn't scratch it. He held his arm up to the window light instead and looked at the edge of it, where the dark crust gave way to skin so new it was shiny and a little pink.
The scrape had been long. He'd gone over the handlebars onto gravel a week ago and peeled a strip of himself off against the road. For two days the wound had wept and stung. Now there was a line of fresh skin filling in from the sides, smooth, the right color, the right thickness. Like the gravel had never happened.
That was the part he kept getting stuck on.
His skin knew how to be skin. Not approximately. Exactly. The new cells coming in along the edge of the scrape were the same kind of cells that had been there before the road took them. Same job, same shape, lined up in the same layers. Nothing had told them what skin looked like. They had just known.
He pressed gently near the healing line and felt how it was already part of him, already alive, already working.
There was a number he'd read once and never been able to put down. He had written it in his notebook months ago. He found the page now, the pencil gone soft and gray at the edges. Three billion. That was how many rungs there were in the ladder inside one cell, the chemical letters spelling out everything a body needed to know about being a body.
Three billion letters. In every single cell.
And to make a new skin cell, a cell had to copy all three billion of them. Then split in two, and give a complete set to each half.
Soren put the notebook down and looked at his arm again.
How long was the scrape. Maybe as long as his hand. To close a gap that size you needed cells. A lot of cells. Each new one copying three billion letters before it could even exist. He tried to picture copying three billion of anything. If he wrote one letter every second and never slept, it would take him longer than his whole life. Longer than a hundred lives.
And his arm had done it in a week. Quietly. While he was asleep, while he was eating cereal, while he was annoyed about not being allowed to swim.
He got a feeling in his chest like the floor of a room dropping away.
Because here was the thing nobody had told him, the thing that arrived all at once. When you copy three billion of anything, you make mistakes. Soren copied spelling words for a quiz and missed two out of twenty. Two out of twenty. If his cells did that, if they got even one letter in a hundred wrong, the new skin would not be skin. It would be wrong. It would be a smear, a mess, a thing that didn't fit.
But the skin filling in along his arm was not a mess. It was perfect. It matched.
He sat very still and did the arithmetic the slow way, the only way he trusted.
Three billion letters, copied. And the body got fewer than one wrong in a billion. He had read that too, and not really believed it, the way you don't really believe a number until you're sitting with your own arm in the window light needing it to be true.
Fewer than one in a billion. He worked it through. Three billion letters, one mistake per billion copied. That was maybe three small errors in a whole new cell. Three. Out of three billion. And the cell had ways of catching most of even those, going back over its own work, fixing what it found, the way he reread a sentence and caught the word he'd doubled.
The copy machine in his skin was reading its own work. Proofreading. While he slept.
He thought about his spelling quiz again, the two words he'd missed, and how his teacher had drawn a small red line under each one. Two out of twenty. And here was his arm, getting fewer than one wrong out of a billion, over and over, in cell after cell after cell, with nobody drawing red lines, with nobody checking, because the checking was built into the thing itself.
Soren had always felt like the kid who needed too many steps. Who reread. Who went back over his work three times when other kids handed theirs in and ran outside. He had felt slow about it, sometimes. Embarrassed, a little.
Now he was looking at the most important copying in the universe, the copying that decided whether you were you, and it worked exactly the way he worked. It went carefully. It went back over itself. It caught its own mistakes before they became permanent.
The thing that made him feel like the odd one was the thing his own cells did three billion times to keep him alive.
He turned his arm slowly in the light. He thought about the cells along the edge of the scrape, dividing right now, each one copying the whole library before it split, each one checking the copy, each one almost never wrong. He thought about how he had been built this way from one single cell, that first cell copying and checking and splitting into two, then four, then a body, then a boy at a window who could wonder about it.
He got the strange dropping feeling again, bigger this time, because it didn't stop with him. Every living thing did this. The dog next door. The grass. The mold on the bread his mother had thrown out. All of it copying three billion letters, or more, or fewer, all of it proofreading, all of it almost never wrong, going back over its own work in the dark, forever, since the very first cell that ever was.
He lifted his arm closer to his face and looked at the new skin until his eyes lost focus, trying to see the place where one cell stopped and the next began.
The itch came back, faint, along the healing line. He let it. Somewhere under the surface, a cell finished reading its three billionth letter, found no mistake, and quietly split in two.
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A science-verified short story for curious kids · Curiosity Land