Jiraiya found out on a Wednesday.
It wasn't Sato's fault, exactly. Or it was, but not in a way he could have reasonably predicted. He'd left the chakra theory book on his desk during the break between classes — Chieru's copy, old and soft-cornered, the spine cracked from years of someone else reading it before him — and when he came back Jiraiya was standing over it with his arms crossed and an expression that suggested a significant internal event was occurring.
"You're reading this," Jiraiya said.
"That's my book."
"You're reading this." He held it up. Not the cover — the page it had fallen open to, which was midway through a chapter on chakra network compression that was, objectively, not beginner material.
Sato took the book from him. "My grandmother has a library."
"Orochimaru reads stuff like this."
"Orochimaru is very smart."
"So are you, apparently." He said it without any particular weight, just dropped it there the way he dropped most things — openly, without ceremony, like a fact was a fact and why would you dress it up. Then he sat down and leaned his chin on his hand and looked at Sato with that expression. The one that meant he was about to propose something.
Sato put the book in his bag. "No."
"I haven't said anything."
"You have the face."
Jiraiya opened his mouth. Closed it. "What face."
"The face you make right before you suggest something that's going to create a problem."
There was a pause in which Jiraiya clearly ran through his recent history of facial expressions and found something that gave him pause. "I just think," he said, slowly, "that if you're reading that kind of material, we should test it."
"Test it how."
"Race through a chapter. Both of us. See who understands it better by the end."
Sato stared at him. "That is not what reading is for."
"That's what everything is for." He pulled his own bag open and took out a scroll — his chakra notes, which Sato had seen before and which were dense and slightly chaotic in the specific way of someone who thought fast and wrote faster and had never fully made peace with linear organization. "Come on. It'll be useful."
"It'll be a competition."
"Useful competitions are the best kind."
Sato looked at him. Jiraiya looked back. The class was starting to filter back in around them, kids dropping into seats, someone knocking something over near the door.
The honest truth — the one Sato didn't say out loud — was that the idea was not entirely without merit. Reading alone was one thing. Having to explain it, defend it, argue about it with someone who was going to push back — that was different. That was the kind of learning that stuck.
Also Jiraiya was going to keep making the face until Sato said yes.
"After class," Sato said. "One chapter."
Jiraiya's grin went wide and immediate. "I'm going to win."
"There's no winning. It's not a competition."
"It's absolutely a competition."
They did it at the river. Jiraiya's place — their place now, Sato supposed, it had become that without either of them formally agreeing to it. They sat on the bank with their shoes off and Sato opened the book to a chapter on chakra flow interruption that he'd already read once but not deeply enough, and Jiraiya produced his scroll and a brush and the particular focused expression he wore when he was taking something seriously, which looked almost nothing like his regular expression and was somehow more alarming.
They read. Separately, quietly, no talking. Just the river and the late afternoon and the scratch of Jiraiya's brush on the scroll when he made notes.
Sato read the chapter once straight through. Then back through the harder sections. He had a habit — old, from his previous life, from years of accounting texts and dry financial theory — of reading things twice before he trusted his own understanding of them. First pass for the shape. Second pass for the detail.
After twenty minutes Jiraiya put his brush down and said "okay."
"I'm not finished."
"I'll wait." He leaned back on his hands and watched the river. He lasted about forty seconds before: "Are you finished now?"
"No."
"Now?"
"Jiraiya."
"I'm just asking."
Sato turned a page. "Be quiet."
He was quiet for almost two full minutes. A personal record, as far as Sato could tell.
When Sato finally set the book down Jiraiya sat up immediately like he'd been held in place by a string that just got cut. "Okay. Chakra flow interruption — the part about pressure differentials in the network. Explain it."
"You explain it first," Sato said. "You finished before me."
Jiraiya's expression went briefly uncertain, which meant he hadn't fully followed that section and was now weighing whether to admit it. He decided yes, apparently. "I got lost at the part about the nodes," he said. "The way they described the distribution — I couldn't visualize it."
"Picture water in pipes," Sato said. "Uneven pipe widths. The chakra moves faster through narrow sections and builds pressure at the junctions. The nodes are the junctions."
Jiraiya was quiet for a moment. Then: "Oh." Just that — a small, clean oh, the sound of something clicking into place. He looked at his scroll and made a new note. "That's not how the book said it."
"The book said it badly."
"Why do they do that."
"I don't know. They always do that."
Jiraiya turned his scroll around and pushed it toward Sato. "The section after that. About the counter-pressure technique. I think I got it but tell me if I'm wrong."
Sato read his notes. They were fast and slightly sideways and took a moment to parse but the understanding underneath them was real — Jiraiya had gotten it, mostly, had felt his way to the right answer through instinct where Sato had gotten there through the second read-through. Different paths. Same place.
"You're right," Sato said. "Mostly."
"Mostly?"
"The last part. You said the counter-pressure dissipates. It doesn't — it redirects. Dissipating it would tear the network."
Jiraiya looked at his notes. "That's what I meant."
"That's not what you wrote."
"I write fast."
"I noticed."
Jiraiya took the scroll back and looked at what he'd written and made a face that conceded the point without actually saying so out loud. He fixed the note. Sato watched him do it and felt something he hadn't expected — a kind of ease. The plain, uncomplicated ease of sitting next to someone who was good at things in different ways than you and finding that the difference was interesting rather than threatening.
He hadn't had that, he realized, in either life for a long time.
They stayed until the light went low and orange and the river turned the color of the sky. They covered half the chapter properly, argued about two sections, got one thing genuinely wrong between them and worked it out by arguing louder until one of them noticed where the logic broke and then admitted it. Jiraiya admitted it faster. Sato took longer because he was more stubborn, which was a thing about himself he knew and found only mildly embarrassing.
Walking back to the village Jiraiya said, "We should do this every week."
"It's not a competition," Sato said.
"Every week," Jiraiya said again, pleasantly ignoring him.
Sato looked at the road. The village lights were starting to come up ahead, warm in the early dark. Chieru would have dinner on. The routine waiting for him, solid and quiet and his.
"Fine," he said. "Every week."
Jiraiya made a sound of satisfaction.
From somewhere behind Sato's ribs the door was warm. Not pulsing, not pressing — just warm, in the way of something that was glad about a thing and didn't need to make a production of it.
He pressed two fingers to his chest briefly, walking, and let them drop.
The village came up around them.