Naruto had never been subtle, but this took the cake.
"What the hell is your problem?" he shouted at Sasuke. "I swear, it's like you literally don't want any friends."
Sasuke winced. "Naruto," he managed, deciding to just go out with it, "you know about my brother, right?" Sakura broke off her attempt at outraged defense.
"Yeah, I remember," the blond shivered.
"Well, the last night I ever saw him, he..." Sasuke hesitated. "There's several stages of the Sharingan, you see. And he had the most advanced – the Mangekyo."
"...and?"
"And he told me that to get it...he had to kill his best friend."
Naruto glared at him, unamused. "You do realize this is the nutjob that slaughtered your clan and Genjutsu-tortured you with watching it happen, right?"
Kakashi coughed. "Actually, Naruto...I can confirm." All three Genin stared at him in shock as he lifted his perpetually-draped hitai-ate, and showed them his Sharingan – which then warped in shape. "See...one of my teammates – Rin – had an unfortunate accident with a Genjutsu we couldn't break." Naruto's horror was palpable, and Sakura looked almost pure green. "It's not something you want, Sasuke," the Jonin said seriously. "There's a price to even use the damn thing."
"And that is?" Naruto managed to choke.
"If you use it too much at once, you start to go blind."
Naruto blinked. Before anyone knew it, a snerk. A few giggles. Then, an outright belly-laugh as the blond collapsed. "What?" Sasuke asked. "What the hell is so funny?!" he roared.
"I always KNEW you were – oh fuck you're all a buncha wankers!" Naruto managed between his chortling.
The rest of Team 7 facevaulted.
So, a lot of people have wondered what the joke is.
In short, there was an old Catholic myth (possibly spread elsewhere) that masturbating (wanking) makes you go blind.