I follow Selis to a queue at one of the many gates leading into the dungeon city. The variety of people here is really wide but I recognise three common body shapes. Fat and rich, thin and fast or big and strong. Furniture must weigh a lot and it all needs to be hauled by hand or cart so there are a lot of the big and strong people walking around.

Going through the gates was easy, we waited for ten minutes after which Selis stuffed some coins in a guard's hand. We walk through the dark brown wall, the same colour as the Peak, and take an immediate right.

I am guided through half the city, Selis occasionally stopping someone to ask them some questions. She is wearing a hat to hide her short blue hair and looks adorable, so answers come freely. I look over to Ket who is walking beside me. He just grins at my questioning gaze. It seems I won't get any answers from him then.

I decide to study the city. The buildings around us are made from a hodgepodge of materials. One building has a sparkling white foundation, red brick walls and black stone pillars. Another is made from an eclectic mix of stone, its walls a wide variety of differently coloured and sized stones. I raise an eyebrow as I see concrete slabs beings used as pavement in some parts. I push a strand of augur inside it and see that it is steel reinforced concrete. So the inhabitants of this world decided to build houses with cobblestones and road with pre-fab walls? I can only shake my head at this. It’s like seeing cavemen using a supercomputer as a club.

“Teach, can you help me with a problem?”

I look at Ket and nod. “Sure.”

We then have the nerdiest discussion I have ever had. He has some problems with integration and differentiation. Me telling him that they are the inverse of the other solves that problem for him. I then introduce algebra to him, replacing set numbers with symbols that can represent anything. I shortly explain how to plot these formulas over a two-dimensional grid. Ket is quiet for a few seconds and starts asking questions about three-dimensional grids. Four-dimensional visualisation has him stumped for a little bit.

“You can also visualise it like an array,” I explain.

“Array? What is that word?”

“You can see a two-dimensional grid like a collection of double numbers. The three-dee version is triple numbers. Add another dimension and you have groups of four numbers for each point on that line.”

“Hang on, lets me test that...” Ket’s eyes sparkle with fascination as he applies this concept. I feel the air around his head buzz with static electricity while his hair starts to rise slowly. He is getting too into this, so I slap him on the back of his head.

“OW, why...”

“You were thinking so hard you could not even dodge a slow slap like that?”

“But I was nearly the...”

“You have some sort of danger detection process running, yeah?”

He nods while rubbing his head. Don't be a baby, Ket, I didn't hit you that hard!

“And you ignored the warning it gave you? What if someone shot an arrow just now?”

He is silent and goes slightly pale.

“Don't think too much if you are not in a safe place and can relax. For you and me, getting lost in the numbers is very, very easy.”

Selis stops a woman to ask for directions and we come to a halt. We seem to be in the trading district, there are a lot of open spaces with goods stacked high in between the chaotically built houses. Every man-made structure in this city must be built with leftover building materials, many small leftovers from big batches of brick and stones gathered together.

Selis moves and Ket and I continue discussing.

“What about arrays in more than one dimension?”

“Before we get into that, be sure to keep data and metadata separate.”

Ket looks at me with a question mark on his face.

“I mean, keep the numbers inside an array, and the array number separate.” He still looks a bit confused so I continue. “On the one hand, we have data, like the number of leaves on a tree. Then there is metadata, like the time and date at which the tree had that amount of leaves. Then is also meta-metadata, like the overall trend of the leaves on a tree mapped over the years. And we also have different types of metadata. I use three types; descriptive for quick searching, structural so I can make sense of the data and administrative so I know what I used it for.”

We walk in silence for a bit longer as Ket mulls this over. Ket is about to speak when Selis walks into a store. I follow her while motioning for Ket to be silent for now. The store is one of the alchemical variety, the walls have some bottles on display. The storefront is rather small, a big counter cutting the room in half. A clerk stands behind said counter with his back to big shelves filled with bottles and ingredients.

Selis takes a small book from her pocket and drops it on the counter. “I found this book in the dungeon, what can I get for it?”

Since when did Selis sound so childish? She talks with the voice of a ten-year-old. I peer at the back of her head but do nothing to blow her cover. I can tell from her bones that she is around sixteen, she is just rather short for her age. The clerk looks at her after glancing at Ket and me. He plasters a smile on his face and the negotiations begin.

“Hmmh, a book is worth a couple of gold, can I look inside?”

Selis nods so the clerk carefully opens the book. He visibly pales as he flips pages. “T-this is… alchemy recipes? Did it drop from a boss?”

Selis nods again. “All that thing dropped after a really tough fight was that book. How much can you give me for it?” She pouts a bit, seemingly displeased with such a small amount of loot.

I can see the clerk scheming, his face displaying micro-expressions that are rather easy to read. I walk forward and place my hand on top of the girls head. “A book is worth a few hundred gold by itself, but the content is the most important. Don't get ripped off, sweetie.”

My grandfatherly smile must look pretty weird on my youthful face, but the clerk pales even further. “Two thousand gold! I will buy this book for two, no, three thousand gold.”

Selis whips her head back around and stares at the clerk. I walk back and continue discussing math with Ket in a hushed tone. Selis continues to wrap the clerk around her finger, staring at him with her big, blue eyes.

The clerk ends up buying the book for four and a half thousand gold, and I discuss matrix multiplication with Ket as we wait for the clerk to come back. He bustles out of the back with a big bag of gold in his hands. He gives the bag to Selis while snatching the book. We are then ushered out of the store while the door gets locked behind us. A ‘Closed’ sign appears on the door window a second later.

Selis smiles while giving the bag of gold to Ket. “You get the first one.”

Ket nods and we follow Selis while she walks to another store. Ket stores the bag sneakily in his ring. He turns to me and we start talking about transformation matrixes, with a small detour to discuss vectors.

Selis continues to guide us through the city, replaying the scene at the earlier shop multiple times. She sells the same book at least ten times, earning thousands upon thousands of gold coins. Night has fallen when she turns to me.

“Thanks, teacher, I’m done.”

Her smile is refreshing. I have an idea of what she is up to by now, so that smile clashes with the image I had of her. Cute and adorable with a mile long mean streak.

“What are their projected losses?” Ket asks.

Her cute smile turns a bit savage. “They won't be able to survive this, even if they had ten more daughters to give away.”

I look at the duo, who seem to be the picture of villains whose evil schemes have come to fruition. “Want to explain it to me?”

She looks at me, once again the picture of innocence. “My parents wanted to sell me, so I sold their secrets. Ket helped me make the books.” She pouts a bit. “Just because I could only read books from very close up does not mean I am stupid.”

Okay then… I know that she comes from a merchant background, I did some research on my disciples before kidnapping them, but I have not looked up the finer details. I decide that this is a worthy cause, so I start digging through my data archives.

Selis is the third child of an average merchant family, her parents wanted to give this unneeded child to a noble to placate him. That’s why she was in jail, to facilitate this process. The books she and Ket made are filled with that family's secret recipes, alongside improved and more effective versions, something that family had built their entire trading business on. Selis just ruined her parent's profit for many years to come with a smile on her face.

The reason I don't like to do this also rears its head though, I involuntarily learned about a number of kittens that were beaten to death, the number of rapes per day in Tower city and some torture statistics. I had to dig through a lot more unpleasant information to get the info I wanted, so I distract myself quickly.

“Are you satisfied now?”

Selis looks up at my question and thinks for a bit. She then nods.

“Good, don't let the past hold you back. You won’t see the road ahead if you keep looking back.”

Selis thinks a bit before replying with a smile. “Thanks, I think I am done with my family now.”

With that, we walk back to the city walls. I try to introduce some geometry and trigonometry concepts to Ket but he will need to process the topics we talked about earlier before truly understanding more advanced math. Knowledge is a like a pyramid, talking about higher level concepts is a lot harder without a solid foundation of underlying principles.

I turn to Selis while we stroll. “Can you control all the elements yet?”

She gives another nod. “Hmmh, lightning was really hard though, I can only make a small spark.”

“It’s fine as long as you understand the basics.” We walk in silence for a few more streets. ”You don't have your glasses or water lenses, can you see well now?”

She shakes her head. “No, but I am used to it.”

“Try to control the water on top of your eye to form a lens, you can see sharp without drawing attention that way.”

Her eyebrows climb upwards as she draws some moisture from the air. She places a single drop on a single eye and I see her correcting the curvature of the water contact lens. She stumbles a bit so I put my hand on her shoulder to steady her. We are nearly at the gates when she smiles brilliantly, both her eyes covered with a thin film of water.

“This is so cool! I can see even better than with the water glasses now!” She blinks her eyes a bit, the water on top her irises wobbling around.

I grin at her delighted expression. “You needed to learn some control before trying this out, accidentally crushing your eyeball is very, very painful.”

She grins back and she walks through a gate with a skip in her step. The guards don't even glance at us as we walk through one of the many gates in the city wall. We leave the thirty meter, brown walls behind us while walking to a collection of big rocks. Once there, I turn to the two disciples.

“I want to expand the area inside the Tree before we start exploring the islands by boat. Do you guys know of a place where I can get a lot of dirt without drawing attention?”

Selis shakes her head while fighting to keep her eyes open, and I see Ket suppressing an occasional yawn as he answers.

“There are deserts far to the south, nobody would miss the sand.”

“Nah, I don't want to travel that far. A sandy layer would be useful though.” That, and my drone spotted sandy beaches at the coast further north.

“Then maybe the western plains?”

I raise a single eyebrow. “How far to the west are these plains?”

“A few hundred kilometres? This is one of the most western cities of the kingdom, there are very few villages further west.”

“Okay, you guys get some sleep while I check that out.”

Ket wants to protest, but I cut him off. “You still need sleep, it is also the fastest way to process new knowledge for braincore users.”

He gives a sullen nod and both touch my necklace. I peek inside and see them both talking a bit more while walking to their houses. The Tree is busily grabbing clumps of dirt and debris that are floating through the endless space, slowly piecing the ground back together. I try to communicate with it for a bit, but it is too busy with its self-imposed tasks, so I let it be.

I look towards the west, lower my stance and jump. I have some dirt to steal.