"You've been making good progress." I stated as I dropped a stack of canvases, the girl's full paint set, and metal cage full of squeaking rats near a grimacing Yakumo. "So as a reward, we're going to be doing genjutsu testing today instead of advancing your other training. I'll still want you to do your daily basic exercises, but other than that..." I waved my hand aimlessly around the large, open training room we'd seized for the day given the thundering rain outside.

"Why so many rats? Is that what you meant when you told my parents you needed ryo for training materials?" Yakumo asked, making a face at the two-dozen rodents I'd purchased from the same ninja 'pet store' I'd bought Satsuki her rabbit from.

I sighed and frowned as I thought again how to properly organize my words.

"So, when I started training you, I asked you to tell me what your techniques were generally capable of, right?" I asked rhetorically, and Yakumo nodded. "Recently, I had to explain some things to... another ninja when he asked what I was talking about."

Obito had, after doing some digging, produced a number of scrolls he'd had me look over and explain to him in greater detail. I'd... gone on a bit of a rant about proper scientific procedure and how these things should be tested and Obito had ended up getting a weird look on his face before we'd gotten into the meat of the discussion and I'd finally understood exactly what was going on. Because whoever wrote those scrolls he dug up?

They were absolute shit at research.

"To cut a very long and involved story short." I curtailed my own thoughts on the matter as I looked at Yakumo. "I took some things for granted when I initially asked you questions about your family's techniques and it would be remiss of me to move forward without understanding exactly what you're capable of in your specialty."

Yakumo nodded hesitantly, still not understanding I think, but willing to follow me down the rabbit hole and see where it went. I reached into the bag I'd brought with me and pulled out a thick notebook I'd been scribbling in over the past few days. "So, we're going to do some tests."

Then the door to the dojo slid open and a damp Tenten stepped in, huffing as she toweled herself off. "Sorry I'm late! The teachers are really pushing us and I had to stay late since I got last draw on testing order."

I turned and nodded. "It's fine. Yakumo and I are just doing some genjutsu stuff today. Go ahead and run through the sets I gave you last time as a warm-up and I'll be with you in just a bit."

Tenten barked an affirmative, then looked around again. "You know, I would have expected Satsuki and Naruko to beat me here even with the rain."

I shook my head. "Satsuki has clan stuff today and Naruko promised to play ninja with Enkai and Konohamaru."

Tenten paused, then nodded and finished toweling off before moving to practice the stances I'd shown her. Satisfied, I flipped open my notebook and grabbed a pen I'd splurged a bit on before tapping the first point I'd made. "So, first off you're going to kill a few of these rats in different ways. Cut one, drown another, and so on and so forth." I paused. "Questions?"

Yakumo frowned pensively. "Can I ask why?"

I nodded. "I'm going to be observing how you channel your chakra while you paint and I want to see what precisely happens with the rats when you kill them as well as what happens when you cancel the genjutsu afterwards."

"Observing?" Yakumo asked as she began to set up her canvas and paints. "But... you don't have any way to see chakra." She grimaced. "No offense, of course."

I shook my head. "The easiest way for me to do so will be to place my hand somewhere on your body and activate a medic-nin technique while you paint. I think it would probably be best if I used the back of your neck, if that's okay?"

Yakumo blushed slightly, but after a moment's thought, she nodded. "That... I suppose there shouldn't be anything wrong with that."

What followed was a series of experiments that told me quite a bit and confirmed an insidious suspicion that had been forming since Obito had tracked me down again to explain his homework to him. As payment, because he was a grown-ass man and could damn well afford to pay me for being his tutor in shit he should already know, I'd requested various long-form techniques and that he ask some very specific questions to certain people on my behalf. Of course he'd given me another one of those weird looks, but had been surprisingly willing to make the deal.

The answers had been illuminating and damning.

The Elemental Nations, quite simply, did not have the scientific method.

Oh, the word 'experiment' was in their vocabulary, that was for sure, but it didn't have the same connotation I was used to. In some ways, of course, this explained so goddamn much about every aspect of their society that I wanted to bash my head against something for not seeing it sooner. Development of technology and chakra techniques wasn't a matter of careful sequential methodology you explained at every step. No, instead it relied on an intuitive grasp of any given subject held only by a genius in that given field and even then they were prone to randomly throw shit at the problem until they got something that kind of worked well enough for their purposes until the next genius came along and refined it further.

I swear I almost goddamn strangled Obito when he looked at me gormlessly and asked what a fucking experimental variable was.

As Yakumo killed the tenth rat and I removed my hand from the back of her neck to deactivate my modified diagnostic technique, I noted that Tenten had moved in to observe our practice now that her initial exercises were complete. Disregarding her for the moment, I moved over to examine the suffocated rodent and reactivated my technique to do a thorough autopsy. On a tray nearby, nine other rats very permanently dead still showed their cause of death despite the technique having been used supposedly being a genjutsu.

I took a deep breath as I laid what was likely the last rat we'd have to kill today alongside its brethren. Grabbing a bottle of pure alcohol, I sterilized my hands before returning to my notebook and beginning to make various notes on the things I'd realized. Nothing conclusive would be put on the paper, of course, but it should be enough to tickle my memory in the unlikely event that I happened to somehow forget this absolute bullshit.

"Did you find something out?" Tenten asked, looking between us.

"Yes." I nodded, continuing to scribble.

Yakumo perked up, looking interested suddenly. "Really?"

I sighed and set aside my notebook and pen before looking up at them. "Okay, so explain to me in your own words how genjutsu work."

The two girls exchanged looks before Yakumo tentatively started talking, as if she suspected there was some kind of trap. "Genjutsu are techniques which, unlike ninjutsu, interact with the target's chakra system itself and largely bypass the physical body. As most illusion techniques are composed of heavily yin-aligned chakra, they use that imbalance to create a sympathetic bond with the target's own yin chakra and manipulate the perception of the individual through that means."

Tenten nodded eagerly and pointed at Yakumo. "Basically that."

I gave her an unimpressed stare and she winced. "I'm not good at that stuff!"

My stare did not abate. "You still need to know it. We'll be adding you to Naruko's review sessions for the next month just to make sure you know your basics."

Tenten clicked her tongue, but merely looked away instead of arguing with me.

That settled, I turned back to a rather smug Yakumo and nodded. "Okay, so that's the accepted explanation and mostly correct, but there's a misconception or two." Her eyebrows rose. "You see, most genjutsu actually use a nearly-identical set of hand-signs somewhere in their long-style formula, but they're so basic that they get edited out when you're taught the technique by someone else. This is because you already know them. It's the bunshin jutsu, the basic clone technique. It's embedded in virtually every genjutsu you'll find that you actively cast on another person." I paused to emphasize that statement. "Some area-of-effect genjutsu don't use the clone as a basis, but they tend to be the ones you cast on a place rather than a person. Given what I just said, either of you care to venture a guess why?"

Yakumo leaned back in her chair, thinking hard on the subject. Tenten looked a bit confused, but willing to toy with the puzzle before cocking her head as something struck her.

"Is it..." The bun-haired girl started hesitantly as we turned to her. "Um... like, when you cast a genjutsu on someone, you're... making a clone of them? Except they think whatever the clone is seeing or feeling is what they're seeing or feeling instead? Like that kind of fuzzy feeling in the back of your head when you use the clone technique to tell your clones what direction to go or what attack to use, but reversed?"

In other words, the clone was a 'universal adapter' template composed of a tiny bit of yang chakra to give it physical form and properly bond with the yin of other people's chakra. That would let the jutsu interface with the person because they were feeling sensations that the illusory clone body overlaid onto them was feeding them.

Yakumo's eyes widened as it clicked for her and I nodded with a genuine smile at Tenten. "Very good."

"But even if the genjutsu I get taught work like that, my own painting techniques don't!" Yakumo blurted out in confusion, a first for the normally-withdrawn girl. "I-I think I know what you mean, Kotaro, about there being a similar base to most genjutsu. I think I could feel it, even if I didn't realize it, but that's why my techniques never felt the same as normal genjutsu!"

"No, they don't." I nodded in confirmation and Yakumo heaved a sigh of relief. Gesturing to the dead rats, I took a breath to explain further. "Your techniques don't have the tiny core of yang chakra common to the basic clone. They are almost completely pure yin-chakra. Because spiritual energy likes to bond with physical energy, genjutsu uses the kernel of yang-chakra to bypass that tendency. Your painting techniques bond your yin chakra directly to a target's yang chakra, effectively substituting the target's own spiritual energy to manipulate their physical."

"That would mean... the reason why fatal injuries don't go away when I remove the genjutsu is because there's no yang-chakra left in the target to revert their physical form." Yakumo frowned as she contemplated what I'd told her, her brows furrowing. "Physical energy and therefore yang-aligned chakra is directly related to stamina, so this is because of my condition, isn't it?"

There was... something in her question, a dark suggestion of certainty that implied I didn't need to answer, but I nodded anyway. "I can't conclusively prove it, but I strongly suspect it is, yes. Or, well, your ability with yin chakra and your weakened physical condition are related, it would be more accurate to say. I don't think it's accurate to say that either caused the other."

Yakumo took a breath and nodded to herself. "I... see." She shook her head, then, looking back to me. "What does this mean for my techniques?"

I drummed my fingers against my thigh. "It means I want to do more testing, but... I have some ideas." Rubbing at my chin, I slowly nodded. "I actually want to try something else first, though. Specifically, I want you to do a painting of a landscape that you can project around us and change the environment. I think I'm starting to get a grasp on how your techniques work on people, but I want to see how they bond to a location that doesn't have yang chakra."

Yakumo nodded decisively and pulled up a new canvas as I turned to Tenten. "While she's doing that, I'm going to be teaching you another move set. You've only got another two or three months until you get your genin team, so I'm going to try and get you as close to mastering the rest of the style as I can in that time."

In fact, I'd probably be putting in a month's expenditure to master both Shii-Cho and Hiten Mitsurugi soon, but what I'd just discovered with Yakumo's techniques more than made up for the delay I was going to suffer from expanding both styles to their full mastery.

Because I hadn't told the girl everything I knew about her personal painting illusions.

There was something else in the chakra mix she was using, something roughly analogous to the clone technique in a normal jutsu, at least in the 'location' of the energy, but it certainly wasn't yang chakra. It felt oily and dark, strange and alien in ways I wasn't sure what to make of just yet, but I wanted to know. I needed to know, especially with the possibilities her techniques held.

Because if I was right...

Somehow the girl had stumbled on a bastardized imperfect version of the Sage of Six Path's Creation of All Things technique that didn't use Sage Chakra.