“I mean, come and fight me!”

“HA???” Kaito marches at Ren with a taunting sneer. “Are you fucking kidding me?”

Ignoring Kaito’s taunt, Ren stands still on his ground, getting himself ready. 

Kaito waits for Ren to break off his joke. He doesn’t look like someone can fight. He doesn’t even stand in a stance, he just casually stands there without a special posture.

“Now, pretend we are in a street fight situation and I was the one who messed with you.” Ren declares. “Now come at me in one strike.”

Kaito scoffs, “Look at you, the boy who gave me his wallet and begged me not to bully him now asking for a fight. Omoshiroi~”

Ren clenched his jaw, that was a memory he doesn’t want to remember, he will make sure Kaito will regret that. “Stop yapping and give me your best strike!”

“I don’t know what this is about! But since you asked me so nicely I shall fulfil it. However, since you are not trained in martial arts I won’t use any Kyokushin move at you.”

“You will regret that.” Ren smirks.

“Here I come!”  Kaito charges Ren while throwing a punch.

If Ren doesn’t know they are there to fight or if Kaito doesn’t give him a warning, it’s impossible for Ren to avoid the punch. It barely passed him and only slightly grazed his cheek.

When Kaito still hasn’t regained his balance from missing the punch, Ren glides to his back, pulling Kaito’s hem on his shirt and hooks it on Kaito’s head from the back. At the same time, using his legs, Ren tripped him. Causing Kaito to plummet to the ground.

“Take this off!” Kaito’s hands scrambled to release the shirt that covered his head and blocked his vision. However, without wasting the momentum Ren grabs Kaito's hands and ties them to his back while using Ren’s tie that he had loosened up before. 

Kaito struggled to let loose, “What??? How can you be so strong???” He fails to shake off Ren’s grasp, especially Ren using his weight to pin Kaito down, pressing his knee on Kaito’s back like a cop just arresting a criminal.

“I may not look like it, but I’m going to a gym too and working on arms strength,” Ren tied Kaito’s hands strong enough to not be easily freed but not too tight that could possibly hurt his wrist.

“You are using dirty tricks and tools! This is unfair!” Kaito complains, writhing like a caterpillar.

“I told you to imagine this is a street fight. No one cares about the rule since there’s no referee. Your first fault is underestimating your opponent and lack of creativity. Speaking of creativity…” Ren helps Kaito to get up on his feet.

“Wh- what are you doing?”

“Today's lesson will be ‘how to enhance your creativity in fighting using physics!’” Ren announces with an elated voice, Kaito can not look at him but he dares to bet Ren is grinning from ear to ear like a Cheshire cat. 

“What do you me-”

Before Kaito can finish his words or recover from his disoriented condition, Ren gets Kaito’s head into the bucket. “Since there’s no safety gear here, let’s pretend this is a helmet.”

“I understand your definition of creativity but how can this teach me physics???”

“Now, listen to me carefully.” Ren begins. “We will learn about force. Force in its purest physical form is any external influence or interaction that will change the motion of an object. Force is most commonly represented and defined by Newton’s Second Law; the force on an object is equal to its mass times its acceleration. When you receive a force, you are receiving a mass that has been accelerated towards you.” 

Without any notice, Ren suddenly punches Kaito in his stomach. It wasn't fatal but hurt enough to surprise Kaito. “WHAT THE HELL IS THAT FOR?”

“You may already know this, but in order to punch harder, you can not only gain the mass of your body. A bodybuilder can have a weaker punch compared to a featherweight UFC fighter. Do you know why?”

Kaito thinks he watched a lot of martial arts tournaments and not seldom found a weaker-looking player wins against the bulkier one, in his analysis the reason is..”The difference in technique?”

“Bingo! To be precise it is in acceleration.” Ren opens the knot that tied Kaito’s hands and releases him. “Now throw a punch again.”

Kaito starts to get the hang of it, therefore he does what Ren told him and throws a punch, this time using his Kyokushin technique.

Using Kaito’s cane, Ren points at Kaito’s legs. “The movement on your leg while your hands are throwing punches is not just a technique created randomly.  That footwork means to generate the acceleration of your punch; same as boxing and other punching required techniques, impossible if your foot just stands still. In conclusion, to gain force we need to harness our mass and speed. So, therefore, in physics, the equation for Force is mass times acceleration.”

“I…See…I don’t know if there’s a logical explanation that way,” Kaito finally gets interested and stops complaining about the bucket on his head.

“The second thing I will teach you is about momentum. The term momentum is defined as the ‘quantity of motion an object has’. Momentum can be thought of as an object’s resistance to stopping.”

Kaito pays a lot of attention to Ren's words inside his dark bucket, and when suddenly he feels someone hugs him from the front, he can’t help being flustered.

Ren obviously doesn’t mean to hug Kaito. He attempts to grapple him and take him down by grabbing Kaito with both arms around his legs while keeping his own chest close to him. However, since Ren never actually practised and only copied from what he watched in tutorial videos, the posture comes out a bit awkward. This is also the reason why Ren covered Kaito’s face with a bucket. He’s grateful Kaito doesn’t point that out though, so he continues his lesson.

“Since the equation for momentum is mass times velocity, being the bigger fighter is generally an advantage due to being harder to move and harder to stop when it comes to grappling. Well, you may be a bit taller but, in body mass, we are in the same weight class, so why do you think I can’t push you back?” Ren asks and waits for Kaito’s answer, still in an awkward position.

“Um..well…since you haven’t eaten lunch yet?”

“Joking again and see how long I’m going to tie you up after this!”

“Wa-wait!” Kaito ponders. “If you suddenly grapple me without any square off of course it would be hard”

“Then how about this?” Ren takes a few steps back before charging in at full speed like a raging bull. Butting at Kaito and success in pushing him back although only a bit. “To be able to maximise momentum in your weight class, improving the velocity of your entry is imperative. Same case if you want to ruin the opponent's momentum.”

“By messing with their velocity?” Kaito asks, now he understands the concept.

Ren hums admittedly. “Throw a punch at me again.” He orders.

“Anyone who heard you would think of you as a masochist, Hasegawa.” Kaito sounds concerned yet still complies.

However, before Kaito can fully deliver his strike, Ren meets his palm halfway. Stopping Kaito’s punch by grabbing them with his hand. “Meeting your opponent's punch halfway will stop their momentum. The result of course would be less fatal and less hurt than when you just stand still and receive it. This is also related to Force theory; mass times acceleration. You stop their momentum, you also reduce the acceleration, which means the lesser force impact you get.”

Just after Ren explains this to Kaito, the lunch bell is ringing. “This…will be the end of today's lesson!” Ren clasps and takes off the bucket from Kaito’s head. "I taught you the concept of Force and Momentum, you should read the theory again from the book I gave you since they might ask about Newton’s theories in the test.”

Ren has bitten back his embarrassment since a while ago. He is eager to leave immediately before Kaito can see his face so he bolts out right away. 

Ren rotates his slightly throbbing shoulder. He barely handled Kaito’s power when he manhandled him back there. He’s undoubtedly strong and perhaps he even still holds his full power at Ren. Ren can win only because Kaito let his guard down and Ren had set a perfect fighting environment in his favour. If it’s other scenarios, there’s no way Ren can even lay a finger at him.