Chapter 15:The Surface Tension

The ruins were filled with the same sound of “Atmospheric Cavitation” and the heavy clack of “Kinetic Fever”,a name given by Rei to his specialized weapon.

“Koga,you’re still pulling your strikes!”Mio roared her Tachi carving and slicing a vacuum arc through the humidity.”I want to challenge the weapon and its user using! Give it some energy that it wanted!”

Rei didn’t respond with words.He merely shifted his center of gravity,redirecting the air pressure around his shoulder.”The recoil of this weapon isn’t worth a sparring session,Mio.You’re seeking destruction,I’m seeking minimal point.”

Click

The sound of a professional shutter cut through the violence.Hiyori stood by a crumbling pillar,her new leather bag over her shoulder,along with that “Axolotl” plushie charm attached to her camera bag.She wasn’t just watching,she was documenting the horizon behind them,the way the golden morning light hit the dust caused by an intense sparring session between Mio and Rei.

“You’re both leaking energy,”Hiyori said calmly,looking at her digital display.”Mio,your left heel leaves the ground when you lunge”

Mio looked embarrassed by her own flaws,”You..you’re right,Hiyori,but it’s kind of hurting me internally”,Mio replied to Hiyori,with an embarrassed tone.

“and Rei,you’re looking at my camera a bit too much.” Hiyori quote on Rei’s performance.

Rei’s face looked a slight red from her comment about his performance.

“Looks like someone has a crush” Mio teases Rei,which Rei questioned if he wanted to use the full strength of “Kinetic Fever” just to beat her after all,she asked for it.

Ten minutes later,Kenji scrambled up the stone steps,panting.”I’m here! I’m… 14% later than scheduled!Statistically,that means my training duration should be reduced to accommodate for the lost–”

Mio’s shadow fell over him.She was holding her Tachi with a terrifying,calm grip.”Actually,Kenji in this ‘system’ ,being late is inexcusable so that means,you just earned an extra two hours of both me and Rei’s drills.”

“Wait..That’s not fair!” Kenji whimpered,dropping his bag.

“It is what I promised to your dad and Mio,Kenji.Now for today’s training,hundred punches and kicking drills to your stance to build up endurance and strength.”.”And 1 hour of my ‘striking training’ hehehe.” Mio cut in Rei's sentence.

“Please have mercy on my soul,” Kenji whimpered.

After the long “Hellish Training”,the group retreated to a nearby waterfall.It was a place of “Laminar Flow”,the water falling into a smooth,plastic-like layer.They sat on the rocks,sharing the foods they brought and bickering over the “best” way to skip stones.

The foods the group brought includes:

• Rei’s food: sandwiches with balanced ingredients for a healthy meal,”perfectly crafted healthy meal” quote by Kenji.”This meal is kind of bland,no personality.However it’s good for healthy people.” quote by Mio,”This looks soft and cute to eat,I don’t want to ruin it by eating it” quote by Hiyori.

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• Kenji’s food:Caprese skewers with a uniform of mozzarella pearls and cherry tomatoes to reduce the variables in every bite,perfect for someone like Kenji.”It looks and tasted good,better than I expected.” quote by Rei,”This meal is alright and decent but small in personality,” quote by Mio,”This meal looks pretty unique and tasted good.” quote by Hiyori.

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• Mio’s food:The “Primal” and “Delicious” quote by Mio, skewers,the skewers included grilled chicken and bell peppers marinated in ginger and soy,perfect for people like Rei and Mio who can get protein sources,anti-inflammatory compounds and essential vitamins.”This tasted like my past self meal routine but it is healthy,” quote by Reizen,”This tasted too spicy,I can’t handle it!” Kenji,drinking two cold sodas while quoting this,”This looks slightly aggressive but I enjoy the balance between spice and the chicken taste.” quote by Hiyori.

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• Hiyori’s food:Bear-Shaped Onigiri or Rice balls with rice shaped like balls and looked like cartoonish bears packed in a wooden bento box,perfect for a photo shoot.”This… this is too cute,” Rei quoted while covering his mouth due to him blushing on how cute the food is while trying to maintain his neutral face,”Finally,a statistically accurate picnic food you would bring on a picnic.” quote by Kenji,”I guess it's good and has more personality than the other two foods those idiots brought.its not boring at least.” quote by Mio.

The best way to skip stones provided by the group:

• Rei’s idea:Treat it as an exercise in fluid dynamics and gyroscopic stability. Select a flat, circular stone and launch it with maximum spin to ensure it remains stable and parallel to the surface upon impact. Aim for an attack angle of approximately 20 degree relative to the water; this specific inclination balances lift and drag, allowing the stone to bounce rather than "plop" or "trip." By minimizing your release height and maximizing horizontal velocity, you ensure the stone skims the surface tension with the least amount of energy loss per skip.

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• Kenji’s idea:To achieve a maximum skip count, you must launch a disk-shaped projectile with a high Reynolds number and a precise attack angle of approximately 20 degree, which minimizes collision time and maximizes the lift-to-drag ratio. Crucially, you must impart sufficient angular momentum to induce gyroscopic precession, ensuring the stone’s orientation remains stable against the asymmetric torque of the water’s impact. By minimizing the incidence angle and maximizing translational velocity, you reduce the energy lost to wave-making and cavitation, allowing the stone to "surf" the high-pressure wedge of the water surface.

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• Mio’s idea:Drop into a deep, rooted stance and coil your core like a loaded spring. You aren’t "tossing" that stone; you’re launching a high-velocity projectile strike. Explode from the hips and whip your arm through a lethal kinetic chain, ending in a violent, snapping wrist flick that drills the stone into a high-RPM spin. Strike the horizon with pure kinetic intent—force the water to obey your power and reflect the stone back into the air. You aren't asking for a bounce; you’re commanding it through explosive force and a relentless follow-through.

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• Hiyori’s idea:Think of each skip as a frame-within-a-frame. Position yourself at a low-angle "worm’s eye view" to compress the perspective and make the skips appear more dramatic against the horizon. Execute a smooth, rhythmic throw to create a consistent visual cadence of splashes, treating the water like a high-shutter-speed stage. You aren't just skipping a stone; you’re manipulating specular highlights and liquid geometry to create a disappearing trail of "ghost frames" across the water’s surface.

The peace was shattered by a sharp high-pitched scream.A small child,having slipped from a mossy rock,was being pulled into the churning “Turbulence” of the lower river.

Rei and Mio moved instinctively before the sound of the splash even ended.

It was a race of “Force Vectors”.Rei used his “Low-Profile Redirection” to skip across the slippery rocks with zero wasted momentum.Mio used a burst of speed,her bokken still sheathed but used as a balancing pole to vault over the rapids.

They reached the child at the exact same time.Rei grabbed the child by the collar,while Mio braced Rei’s back against the current,her bokken dug into the riverbed as an anchor.Together,they were an unbreakable border against the water.

Once the child was safe on the bank,shivering but not injured ,he looked at the two terrifying teenagers,but he didn’t seem to be scared of either of them. Hiyori came and asked the boy,”Are you alright?” The kid said “Yes!”

Kenji thought to himself,”The chances of the boy surviving without Rei and Mio is near zero! Thankfully,both of them arrive which increases the chance to a near hundred!”

The kid turned to his two terrifying heroes,”Thank you!” The boy chirped,wiping his eyes.”Are you two best friends?You both move at the same speed!”

"No," Rei and Mio said in perfect, unintended synchronization.

"We're rivals," Mio huffed, crossing her arms. "I'm just waiting for him to drop his guard so I can win against him.”

“We are… functional associates,” Rei added,though he didn’t pull away when Mio nudged his shoulder.

Hiyori snapped a photo of three of them–Rei,Mio and the kid.”Whatever you call it,the ‘Physics’ looked like a team to me.”

As they headed home, the sun setting behind the mountains, Rei felt a strange sensation. For the first time, his heart rate wasn't elevated because of a fight or a "Kinetic Fever" recoil. It was just... steady.

But as Hiyori walked ahead with Kenji and Mio, Rei stopped for a second, looking back at the waterfall. The "Surface Tension" of his life felt perfect. And as any physicist knows, when tension is at its highest, it only takes one small grain of sand to make the whole thing snap.

Back at the Sato household,a message appeared in Rei’s phone.Rei took a look and put it down instantly,a crimson red appeared in his cheeks.

Hiyori:Hiii Rei! You look like a cool hero for saving a kid and good deeds like that won’t go without an award. How about we go out to a Field Observation! Just the Two of Us!If that's even possible… Goodnight Rei!

Author's note

✍️ Fun Fact:

When Rei and Mio save the child, it’s the ultimate proof of their Rivalry vs. Teamwork. Rei is the "Grip" (grabbing the child) and Mio is the "Anchor" (bracing Rei). In physics, they created a Stationary Support System.