A few days later, Sengoku sat cross-legged in the center of his dimly lit stone house.
Scattered on the floor before him were the shredded, lifeless remains of several thick rubber balls. They looked like the hollowed-out husks of strange fruit. These were the trophies of his brutal, borderline masochistic training regimen over the past few days.
He had blown past the water balloon stage with terrifying speed and had now systematically ruptured every rubber ball in his possession.
As the last wisps of azure chakra faded from his fingertips, Sengoku opened his eyes. He stared at the torn rubber fragments, his brow furrowing in mild dissatisfaction.
'It's not enough.'
The familiar sensation of his chakra control leveling up had completely vanished. Bursting the final rubber ball had felt almost effortless. The material had reached its physical threshold; it could no longer provide the structural resistance required to push his shape manipulation further.
Even the salvaged puppet joint he had bought weeks ago was now utterly useless, too fragile to serve as anything more than a paperweight.
His training had hit a hard bottleneck. The academy's standard exercises, like tree climbing, had long since become obsolete for someone of his caliber. Now, even the second stage of the Rasengan training—drawn from his fragmented memories—had exhausted its utility.
So, what was the next step?
He briefly considered acquiring higher-level ninjutsu scrolls, but reality quickly crushed that idea. He was a penniless orphan; his monthly village stipend barely covered his basic food rations. Requesting advanced materials directly from the academy was equally out of the question. If he exposed his monstrous learning speed and his attempts to reverse-engineer an A-rank jutsu, he wouldn't be hailed as a prodigy. He would be flagged as a high-risk anomaly, dragged into an interrogation cell, and likely end up dissected on an Anbu operating table before sunset.
His gaze drifted aimlessly around the sparse room, eventually landing back on the broken puppet joint. It was made of low-grade iron.
A sudden thought pierced his mind.
Rubber had a specific tensile limit because it was porous and flexible. But what about metal?
If forcing chakra through a rubber ball acted as a whetstone for control, then trying to penetrate a solid, dense sphere of metal would provide exponentially more resistance. The structural pushback from solid iron would easily be tens, if not hundreds, of times greater than rubber.
The idea took root instantly.
While finely crafted ninja tools were exorbitantly expensive, Sunagakure was situated in a desert rich with raw mineral deposits. A crude, unprocessed chunk of scrap iron should cost next to nothing.
Without a second thought, Sengoku stood up. He quickly swept the shredded rubber into a corner, grabbed the small pouch of spare coins he had meticulously saved, and pushed open his heavy stone door.
Finding a weapon shop was simple enough. He navigated the winding, sandy streets until he spotted a modest sandstone storefront marked by a faded canvas depicting a shuriken.
Inside, the air was dim and heavy with the sharp scent of machine oil and cold steel. The shelves were packed with an array of weaponry, ranging from pristine kunai to chipped blades awaiting repair.
The shopkeeper, a quiet, middle-aged man with a cloudy, scarred left eye, barely registered Sengoku's entrance. He remained slouched behind the counter, looking thoroughly apathetic.
Sengoku walked straight to the counter. "Do you have any solid iron balls?" he asked evenly. "No special forging or treatment needed. Just raw, dense, and heavy. About the size of a handball."
The shopkeeper raised an eyebrow, mildly surprised by the odd request. He sized the young academy student up for a moment, but didn't pry. Wordlessly, he turned and began rummaging through a wooden crate of discarded scraps in the corner.
Clang. Clatter.
A moment later, the man pulled out a dark, oxidized sphere and dropped it heavily onto the wooden counter. Thud.
It was about ten centimeters in diameter. Its surface was rough, dotted with casting burrs and a dull, blackish-gray sheen. Sengoku placed his hand on it. It was freezing cold and incredibly heavy, radiating a dense, unyielding solidity.
"Melted from the offcuts of a ruined mold. Mostly used to weigh down tarps," the shopkeeper grunted. "The iron quality is garbage, but it's as solid as they come. One hundred Ryo."
It was exactly as Sengoku had calculated. Because raw ore was plentiful in the Hidden Sand, the material cost was practically negligible.
Sengoku handed over the coins. He tucked the heavy iron sphere into his jacket, the cold metal pressing firmly against his chest, and hurried back to his house.
Returning to his quiet sanctuary, Sengoku resumed his cross-legged position on the floor. He placed the raw iron ball in the center of his palm. The rigid, freezing texture was a jarring contrast to the pliant rubber he had been handling for days.
He took a deep breath, closed his eyes, and gathered his absolute focus. Deep within his coils, his chakra stirred, flowing steadily down his arm and pooling in his hand.
However, the moment he attempted to force that chakra into the iron sphere—expecting it to penetrate as it had with the rubber—he was met with a violent, suffocating wall of resistance.
It felt like trying to force a raging river through a monolithic dam of solid steel. The internal progression of his chakra slowed to a torturous crawl. Every microscopic millimeter of penetration required a massive, exhausting exertion of pure willpower.
Unlike the porous nature of rubber, the hyper-dense atomic structure of the solid metal fiercely rejected the intangible energy. The resistance was staggering—dozens of times stronger than anything he had faced before.
Fine beads of sweat rapidly formed on Sengoku's forehead.
'Yes! This is it!'
This was the exact overwhelming pressure he needed. It was the perfect whetstone to shatter his current limits and elevate his control to an entirely new realm.
Abandoning all thoughts of a quick success, Sengoku sank his entire consciousness into the grueling task. He slowly, methodically fed a continuous stream of chakra into the iron.
The process was agonizingly slow and incredibly draining. His chakra felt like it was sinking into a bottomless ocean, returning almost zero physical feedback.
Time slipped away in the absolute silence of the room.
Yet, beneath the surface, Sengoku could clearly perceive what was happening. Under the crushing, claustrophobic pressure of the dense iron, his chakra was being forcefully compressed and refined. The metal acted like an unforgiving filter. Every minor flaw in his shape manipulation, every tiny leak in his output, was violently exposed by the iron's resistance, forcing his mind to automatically correct and tighten the flow.
The efficiency of this training method was beyond anything he had imagined.
Furthermore, the sheer durability of the solid iron meant he didn't have to worry about accidentally destroying his training tool anytime soon. He could pour his full power into it without holding back.
Hours passed. It wasn't until his mental stamina was pushed to the absolute brink of collapse that Sengoku finally cut the chakra flow.
He opened his eyes and exhaled a long, shaky breath, a faint puff of white mist escaping his lips. He looked exhausted.
In his palm, the dark iron ball remained perfectly unchanged. It was just as cold, hard, and solid as when he had bought it, showing absolutely no external signs of the violent energy it had just endured.
But inside Sengoku's meridians, the sensation of his chakra manipulation felt sharper, tighter, and more profound than ever before.
He gently brushed his thumb over the rough, oxidized surface of the metal sphere, treating the crude scrap like a priceless artifact.
Starting today, his daily tree-climbing exercises were officially retired. With this iron ball, the refinement of his chakra had entered an entirely new frontier.
Sengoku placed the heavy sphere on the floor beside him. He slowly opened his empty right hand, staring at his palm as a new, ambitious thought began to take shape.