"What does it mean to say I can breathe it if it won't power my body?"

"There's a sort of fire inside us which would die. You could pull it into your lungs, it's not even poisonous at all, but it wouldn't sustain you. You'd drown without a drop of water in sight, because nitrogen can't be burned, or rather, it can't burn things like oxygen does. A fire is... The interaction of oxygen, which burns things, and things that can be burned, which releases energy. Things that can burn, and the oxygen in the air, have a certain amount, and things that have been burned- Ashes, and the carbon dioxide fires and breathing both produce- Have less. Think of it like a big ball on a hill. It's happy to sit still, but if pushed hard enough, it'll roll down the hill. But only if something pushes it, which is why things aren't on fire all the time, they need a push to light up. As for air itself, it's a mixture. Nitrogen, this stuff," he taps the frigid beaker, "Is the bulk of it, but oxygen is a considerable portion too, with small amounts of a few other things."

"So, from my perspective, I'm feeling very convinced that nitrogen and oxygen and carbon dioxide exist and act as you say, but I'm not feeling very convinced that they're components of ordinary air outside this shop, or that things you sell me to manipulate them won't turn to stone with the sunrise."

"At some point this stops being my problem. Would you like a free sample of something to see if it works tomorrow morning."

"Yes, I would. I thought you were planning to leave before then."

"I don't entirely control when I leave, though everyone being so skeptical and doing so little business makes it likely I'll go sooner than later. Here, have a lighter. The part where they make fire is completely mundane, the part where you can control the fire is magic. It's got some oil-like fuel in the bottom there, when that's gone it's done, not really refillable."

"Thank you. If you're still here in the morning, I'll see you then!"

Well, at least it gives him time to work on his clockworks.

The enormous wagon is still there the next day.

He gets some visitors overnight but they're mostly just there to gawk.

But in stories, when mysterious things like this show up in the evenings, the magic is gone the next day. Teru's there just after sunrise, and so are a handful of other people, including a couple others who visited yesterday.

He has relaxing guitar music playing in the background today.

"Welcome one and all. Let me know if you need help finding something, and mind the rules."

Teru looks at the fanciest-looking things first - she'll get to everything, eventually, but given her complete lack of expertise it seems like as good an order as any. The other woman who came in with her starts looking for the cheapest things.

A man dressed in work clothes, but very well-made and relatively new work clothes, asks about a cake he heard might be sold here.

There's lots of shiny metal and glass things around! The giant food boxes with big glass fronts are all cold or even frozen inside. There are cooking implements too, including a 'meat printer'. In the crafting section, some of the most expensive items are a needle of infinite thread, a whole series of tools that will half move on their own if held meant to help people learn, self-cleaning paintbrushes, some kind of elaborate workstation with mysterious lights and little black rectangles and colorful metal wires, firedrake leather that looks like some sort of very thick snakeskin and boasts extreme durability and protection from fire.

The cheapest things are items like bread, loose fruit and vegetables, simple items in cans and shiny wrappers for food. In the crafting section, it's more like stacks of wood rods (the kind used for wicker), ordinary paint, packs of stickers and pencils and paper, and assorted lumber, nails, screws, and other fasteners.

"The Immortal Cake is a high-tier and expensive item that could certainly give a physical boost to a group of soldiers. It can be split among many, in fact I generally recommend that, and refines the whole body into a slightly superhuman state. Less exhaustion, less age, healing more cleanly, strength and flexibility, sharper senses... I don't know your full situation, of course, but it may be less efficient for you than something like good leather and steel since it is the single most expensive item I carry regularly. I also have more mundane campaign food that will not spoil for decades and is light and waterproof."

"Hm, when you say you have food that 'will not spoil for decades', is that assuming pests don't get at it or is it immune to pests?"

"Thick metal cans or heavy plastic sealant, still reasonably open-able. Here, let's go take a look- Like this, see," He indicates a row of canned fruit. "My trail food is a bit further back, but you see that tab, you just pull on it to open it up and there's fruit in heavy syrup inside. Just open and eat. You could even melt down the cans later, though it's hardly worth it for most people I sell to."

"Hm." He checks some prices. "And how many people do you recommend splitting the cake among?"

"At least five, but past about a hundred you'll start to hardly notice the effects. It works best if eaten immediately after intense training."

"What would count as intense training?"

"Running twenty miles, sparring for hours, a long day of hard labor, just generally pushing your body close to its limits."

"What if someone were... ill or frail or otherwise unable to do those things?"

"It would still help, but... Not the intended use, to feed it to someone already ill or frail. The Immortal Cake purifies and tempers a body, like a smith working on metal. If the metal is poor, it will hardly be any better afterwards. Such a person may benefit more from an Apple of Elysium."

"What is that?"

"Grown in Elysium, a magical land of comfort and rest, and having absorbed some of that essential essence- They won't heal you, but they'll offer a reprieve from almost any ailment. For a while, a week or two to even several months depending on the severity of the malady, the worst of it will vanish. Pain fades, weakness abates, failing sight can even return somewhat. But not forever. Eventually, it will return, and each apple after the first will help a little less."

"...Hm. And if you take that window to train hard and eat cake it won't help much because it's... not working on the same sort of thing?"

"I haven't ever actually heard the results of feeding Apples of Elysium to someone, getting them to exercise, and then giving them Immortal Cake! Elysian stuff is inherently temporary outside of Elysium, but Immortal Cake is imitating techniques I don't actually know for permanent improvements. I know that physical therapy done under Elysian influence helps an appropriate amount even after it fades... I'm not certain, but I think that would be better than not using an Apple of Elysium, at least."

"I might want to get the materials for that, then. Are the apples also divisible?"