The first time Sasuke really notices Naruto is during their Academy class’s unit on sealing.
To be fair, it would be kind of hard to miss him after the way he shoves so much chakra into the exploding tag he’s supposed to be making that he blows up the back half of the classroom and launches himself about twenty feet into the air. A couple of the other kids are a bit singed, and they end up having to share a room with another class until the terminally overworked Mokuton user can fix the wall and ceiling, but Naruto wanders back in the next day looking sheepish and entirely unharmed.
Since Sasuke saw him take the brunt of the explosion, this is worthy of note. So is the fact that he managed to shove enough chakra into the tag to burn out the safety seal that was supposed to keep things like that from happening.
Under the cover of Iruka-sensei’s yelling, Sasuke looks down at the diagram on his paper, a jutsu that his father dismissed as unreasonable and unusable given normal human limits, and thinks, Huh.
“All your old designs?” Shisui says with some confusion when Sasuke tracks him down after class. He rocks back on his heels, eyeing Sasuke a little warily, and swipes ineffectually at an ink-stain on his cheek. “Yeah, I still have them—I wouldn’t throw them out after you gave them to me for safekeeping, brat.”
“I need them,” Sasuke says with determination. “All of them.”
Shisui blinks, then tilts his head, studying him for a moment. “You know the amount of chakra they need would kill most jounin,” he says, though it’s too curious to be an accusation. “I think even Sarutobi-sama might have trouble with some of them. You’ve got a habit of thinking big, kid.”
“Are you going to give them to me or not?” Sasuke demands crankily, because he’s got curfew in three hours and he still has to pick through his plans to find the easiest one and find Naruto.
“Of course.” Shisui sounds offended that he’d think otherwise. “They’re your designs, Sasuke. Check the hall closet, top shelf—Tenzō!”
Shisui's boyfriend takes one look at the manic light in his eyes, the scrolls scattered over the breakfast table, and the ink smeared across his face, then blanches and turns right back around.
“No no no!” Shisui lunges after him, grabbing up his scrolls and vaulting clear over the table. “Tenzō, no, come back, I swear this one isn’t like last time, this is a brilliant idea, you will be blown away!”
“Urgent mission, ANBU called, I have to go!” Tenzō calls over his shoulder as he bolts.
“Tenzō! That wasn’t even a convincing lie! Come on, you're the only one in the village with Mokuton, you have to try this or my brilliance is wasted, it’s a gorgeous jutsu! Tenzō!”
Sasuke rolls his eyes as the yelling fades behind him and heads for the closet. The box with the jutsus his father rejected is right where Shisui said it would be, and Sasuke clutches it and thinks a little gleefully about the potential of even one of these jutsus, provided he can get Naruto to agree. The destructive power. The awesome might. The possible explosions.
(Somewhere deep in R&D’s basement Mikoto pauses in the middle of creating a devastating combination jutsu, and turns to smile fondly at the picture of her youngest son on the wall. She’s so glad he got more of her proclivities than Fugaku's, even if her husband does tend to complain about the property damage more frequently since she introduced him to the wonders of explosive chakra techniques.)
Naruto is just heating up water for his nightly cup of ramen, halfway through trying to factor a new jumpsuit into his monthly budget while still having money for Ichiraku’s, when there's a knock on his door. A little wary—because Hokage-jiji usually warns him when he’s going to drop by, and there's no one else who visits Naruto—he pulls it open, and finds himself face-to-face with the weird nerd who spends pretty much all of their time in class drawing on big scrolls. Iruka-sensei yells at him to pay attention almost as much as he does Naruto, which automatically makes Naruto like him.
But he and Naruto have never even spoken before, so Naruto has no earthly idea what he’s doing here.
“Hello?” he asks.
The boy thrusts the shoebox he’s carrying at Naruto and says, “Your chakra reserves are amazing will you try my jutsus?”
This is, Naruto learns much, much later, the Uchiha equivalent of a marriage proposal.
Even if he’d known, he probably still would have lit up with glee and cried, “Yes!”
“Not a word,” Mikoto tells her husband as she ladles miso soup into five bowls instead of the usual four.
Fugaku rolls his eyes at her, ignoring her warning to offer, “I don’t think Kushina would have liked anything more than for you to take in her son.”
“And now we have an excuse,” Mikoto says triumphantly, waving the ladle like it’s a weapon to stab Danzō and the other Elders with. “We couldn’t approach him but they didn’t say anything about Sasuke now did they? Hah!”
“I think,” Fugaku says dryly, casting a glance at where Sasuke is looking halfway to manic as he explains one of his jutsus to a beaming Naruto, “that the Elders are very shortly going to have much bigger things to worry about.”