It was three days later when Nara Kazuki finally saw Minato Namikaze again.
Only then did Kakashi tell him they were invited to dinner at the Fourth Hokage's home.
"I mentioned your steam train idea to Sensei," Kakashi said as they walked. Kazuki nodded—he figured as much.
But his mind wasn't on that right now. His attention was focused on the spinning chakra orb forming in his palm.
He was working on the Rasengan.
As Kazuki practiced, something from his previous life came to mind—a vague memory of a physics teacher explaining centrifugal force. Supposedly, when an object rotates fast enough, that force can rip it apart.
What was that term again? Something about critical rotation speed…
He couldn't remember. He'd been a liberal arts major in his last life—science was not his forte. He knew some basic buzzwords like the Tyndall effect, and not much more.
Still, he remembered the gist: every material had a maximum rotational tolerance. Within that limit, you were fine—but push past it, and things fall apart.
So… would chakra follow the same rule?
He figured the Rasengan would be the perfect way to test the theory.
And now, it was finally taking shape.
"The essence of Rasengan lies in understanding rotation," came a familiar voice. "But the faster it spins, the harder it is to control. Start slow."
Kazuki looked up—Minato had appeared next to him at some point. Startled, he glanced around and realized they were already standing outside Minato's house.
He'd been so deep in thought, he hadn't noticed the walk at all.
Minato gave him an encouraging smile.
Kazuki lowered his gaze again and followed Minato's guidance, trying to form the Rasengan more deliberately. The first time Minato had explained this, Kazuki couldn't understand it—he hadn't grasped the idea of optimal spin.
Too slow, and chakra would just dissipate. Too fast, and it became impossible to control. When it vibrated at high frequency in his palm, it was like holding a high-end massage gun set to max intensity.
Actually… the Rasengan probably put that kind of gadget to shame.
Sometimes Kazuki thought this technique would be a big hit for certain niche groups—though it'd spike the water bill.
But after repeated practice, he finally managed to produce a decently stable sphere of spinning chakra.
[Rasengan: 0 / 2000]
A progress bar appeared.
Kazuki's eyes lit up. That meant he had officially succeeded in manifesting the Rasengan—it was now part of his skill list.
Overcome with excitement, the Rasengan lost stability and burst apart. But Kazuki didn't care. Now that he'd succeeded once, refining it further was just a matter of repetition.
Minato led them inside for dinner. Kushina was still cooking, so while they waited, Kazuki and Kakashi went to check on young Naruto.
The boy was still just a toddler—but his bright, alert eyes already had spark.
Kazuki scratched his head.
A terrible thought crossed his mind.
Everyone knew Naruto and Sasuke were reincarnations of Ashura and Indra, the sons of the Sage of Six Paths. Which meant… in some twisted metaphysical way, they were actually brothers?
If you did a DNA test right now, would their genetics even make sense? Would they register as Minato's biological son? Or would some eldritch chakra anomaly show up and void the results?
His gaze drifted blankly, lost in this metaphysical hellhole of a question.
"…Kazuki?" Kakashi nudged him.
Snapped out of it, Kazuki quickly abandoned that rabbit hole. Fortunately, Minato didn't seem to notice his inner turmoil, instead asking about the train project.
If Minato had known what Kazuki was really thinking, he might've hit him with Flying Thunder God: Second Step out of sheer disbelief.
"It's a steam train we encountered in the Land of Snow," Kazuki began. "Much faster than a cart, with way more cargo capacity. If we build a line through the Fire Country, it'll massively improve trade and supply lines."
He kept it simple.
Minato studied the schematics seriously, frowning slightly. Kazuki noticed his expression and couldn't help thinking: being Hokage really wasn't glamorous. You had no time to enjoy anything. Always worrying. Always calculating.
He'd rather be a clan head. Good food, good drinks, no paperwork.
Then again, the authority levels were completely different.
"This is… very promising," Minato finally said. "But it would be difficult to defend. Once completed, the rail line would be vulnerable to sabotage. Maintaining it would be a nightmare. And enemy shinobi could easily disrupt logistics."
He wasn't wrong.
From a village perspective, the train brought clear benefits: economic growth, reliable supply chains… but also massive costs. And with how nimble ninja were, destroying a few key segments would bring the whole line to a halt.
Unless you could somehow guarantee its security, it might not be worth it.
Kazuki grinned.
"Right now, we're the only ones with this tech. Which makes us a target. But if everyone has it, then everyone has to watch their back."
Minato raised an eyebrow.
Kazuki continued, "That's probably why fire trains only show up in the Boruto era. The world was too chaotic before—only when things settled down could technology really advance."
Indeed, in Naruto's reign as Hokage, the world became far more peaceful. Without constant wars, there was finally room for innovation.
But Kazuki had his own solution to the train conundrum.
And he wasn't done yet.