"It replenishes over time, up to a personal limit which varies a lot between people but usually stops growing by age ten or so. Once someone has a reasonable capacity, skill in using it becomes much more important. ...if you meant 'where does it come from' on a deeper level than that, in terms of physics, I don't think the answer is known but I'm not an expert. As far as is known, animals and natural processes don't produce mana and I've never heard a good explanation for that beyond saying that mana is a property of souls. ...and I should have mentioned, not everyone is a mage; maybe one in four or five hundred is, one in several thousand once you filter for an actually useful amount of mana and degree of skill."

"Huh. I think something like a quarter of people are smart enough to be wizards, if they try, maybe more if we were better at teaching it or something, it's just kind of expensive to get started so mostly only the very bright invest in it. Anyway, I work in the way I described, so even if I turn out to be far more allergic to the sky fireball than expected and the first village we come to starts throwing rocks at me without waiting for me to make sudden movements, I will probably be able to go back down without you, due to having seen and cleared the route and also fought anything that got in the way."

Tanya is unsure how much she should take that literally and how much is the bravado and/or naivete of a young woman wanting to escape her hometown! Maybe she should angle for an introduction to Belmarniss's mother, in order to ask her discreetly, except she can't trust her because she is presumably the person who wants Belmarniss to stay here and get married...

"Your whole population has mage potential but only the top quarter studies it? That makes sense; if we had so many mages and also didn't have a war going on most people would probably choose other professions, and there would definitely be a shortage of casting implements and qualified teachers, at least for a long while." They don't seem to be very technologically advanced or to have a particularly high quality of life, but you need a much bigger population than one small city to produce the caliber of researcher who'll invent spells of real economic significance, not to mention a lot of R&D funding. 

"So - the bottom line is that you need to do work to prepare each instance of a spell ahead of time, which means you need to choose what to prepare for, and also you can only have so many prepared at once? How many is that?"

"...you've misunderstood something. Far fewer than a quarter of drow study to be wizards. A quarter is my guess at how many would succeed at hanging first-circle spells if they put in the work but most people are farmers and some people fail at becoming wizards for reasons other than not having enough raw intelligence. You also need to be able to get enough sleep reliably, you can't have shaky hands or a stutter, you have to tolerate a lot of math, and if you drop out to do laundry as soon as you've got your first cantrip that might be a smart thing to do but it's not going to make you a first-circle wizard.

"My wizard spells need preparation ahead of time, yes. My sorcerer spells don't - that's why I can be so profligate with the Comprehend Languages, there's only so much opportunity cost there. ...I can just give you numbers but I'm not sure they'll do you much good since you don't know very many spells that exist."

...yes, that's an obvious mistake in retrospect, a quarter of society spending months or years training to be mages is far more than could be supported when most people are farmers. (Even in Germania, a solid one-third of the population works in farming or farming-adjacent jobs.) 

"I see. And your 'sorcerer' spells are still limited to being cast only so often. Is that not because of running out of mana or some equivalent? Do the more complex spells take more, or is that a separate issue of skill or some other factor? I would appreciate some numbers, I'm not even sure what order of magnitude we're talking about here. ...I can also wait for the class if you'd rather, but it might need to start with a basic overview for me to understand it."

"I could imagine someone modeling sorcery as running on some unifying substance but I don't think it's a good model because spells are discrete and you can't use up lower circle slots to constitute a higher circle one unless you're already really close to having the higher circle one in its own right. You could think of it like having a certain number of arrows of different types that I can shoot, if you could transmute arrows into equivalent quality arrows but not into better arrows on the spot. And if there were also unlimited zeroth-circle arrows, because you can catch a cantrip and re-cast it, which is why I never run out of the one I use to avoid tripping that you've seen me do a bunch."

Oh, is that what she was doing. (A bookish young woman who is prone to tripping is a bit of a stereotype but probably not an inaccurate one?)

Tanya takes a moment to puzzle out the arrow-metaphor. "So you can cast a fixed number of 'sorcerer' spells of each group, the spells being grouped by some mathematical property. One group contains the spells that can be cast any number of times" - presumably up to some mana limit, but a very weak spell like the local lights really can be cast all day long by even a weak mage. "And the same for 'wizard' spells, except you need to prepare specific ones ahead of time. How many times can you cast limited-use spells, then? And how useful are the unlimited spells, for protecting yourself or fighting? ...which reminds me, you said the journey might last two weeks and would certainly last days, we can carry enough food but do we expect to find water en route?"

"We'll want a bag of holding for the trip for that reason. Though a couple weeks is a high-end estimate. I know a lot of cantrips, there's one I don't have but know exists that I'll want to get ahold of before we go which'll keep the sky fireball off my case. Most cantrips are not very combat-oriented but I have Ray of Frost, which I can do as a sorcerer -" She zaps a bit of the floor with it. "I also have a technically-not-a-spell bit of sorcery, a little stronger but not much and not cold-themed, which is limited per-day..."

Belmarniss of course knows all the spells she knows and how many of them she can do in a day and does not mind telling Tanya most of this information.

Tanya has no idea how much actual damage these spells can do when they're being used for real. Presumably a lot more than that, you wouldn't want to freeze half the room as a demonstration! 

"To be clear, I can carry around forty to fifty kilograms while flying, which is more than enough for our water budget. Up to a hundred kilograms for a couple of hours, if I really push myself. But it would need to be in a rigid container, or at least a stack of several containers. My flight spell is optimized for moving the caster and I can only do crude pushes on other objects; if I need to, for example, fly you up a cliff it would be easier to lift a small platform you're standing on." Mages can evacuate other mages under fire with a fireman's carry but they can't do anything else useful at the same time, it's almost as tiring as the real thing, and it tends to leave the person being carried with extra bruises. "And if I'm carrying something like that, I'd be much slower to react to a sudden attack, especially if it needs to be set down carefully. There's also a risk the container would be punctured in fighting, so if you have another solution it's definitely worth pursuing."

"Bags of holding are bags that have more room inside than outside, they're very popular. They're heavier than they'd be if they were just normal empty bags, but they don't get heavier when you put stuff in them. It's also possible there'll be water en route and Detect Poison's a cantrip so we'll be able to tell if it's safe to drink but it's advisable to bring some."

"That sounds incredibly useful! We don't know how to do anything like that. How well does it scale?" Tanya has a brief vision of carrying an artillery division in her bag as she flies.

"You can't nest them, and the ones that hold more also weigh more, and of course they require wizards trained in crafting wondrous items and a fair whack of spellsilver to make, which is why I'm going to need to go to my grandma for money. But they're pretty cool. Also in extremis Ray of Frost does condense moisture out of the air usably."

"That makes sense. I can also condense water but only if it's fairly humid, I wouldn't want to rely on it."

"It's not usually super dry down here but yeah, it would definitely be annoying. If we had a cleric along, they can create water as a zeroth-circle spell, but arcane casters can't and I don't really want to try to get a cleric."

That is a very understandable sentiment!

(Do the local clerics have a spell to make water come out of the stone? It sounds very useful for an underground city with no rainfall. What a pity useful spells are hoarded by the church. Tanya doesn't even want to know what kind of god they worship.)

 "It seems we can ensure our water supply without that, yes. ...I think I'm back to general questions about the world, which are not urgent. I expect you also have some questions for me?"

"Yeah, I don't have a good sense of your usage limits except that if it all runs on the same thing presumably it all trades off against all of it?"

"That is correct. There is a unit measure of mana but I'm not sure how to translate it, it is annoyingly not tied to the other fundamental physical units... Mana is a limit on casting many powerful spells very quickly but over longer periods of time the more usual constraint is tiredness and loss of focus. If you have to cast all day that's very many opportunities to get something wrong, and losing control even once can be catastrophic, either in itself or because it gives an enemy an opening or just because you fly into something. So combat mages have to be very careful and know their limits well. ...it helps a lot that we operate in groups and not as solo 'adventurers', we rely on each other and switch out as necessary. I of course can fight by myself, but I lose a lot of effectiveness this way. More because of being in a cave."

"I very much hope tiredness won't come up as a constraint, I should either eliminate the target or retreat long before then, but if we're pinned down for a long period of time and I keep using magic but can't break out for some reason, that would do it. As for mana, I can't use all I have on a single spell or two anyway, because the individual spells are also constrained by my orb. If I simply kept firing as powerful a spell as the orb allows as quickly as I could then I would run out of mana before I grew tired, after - two to four minutes, depending. But those spells would bring the roof down on our heads long before that, so it shouldn't come up in practice."

"Oh, adventurers are not usually solo, groups of like fourish people is normal. What does the orb do?"

"It's an example of a category called casting implements. They form the mana into specific spells that are built into them. Freeform casting is possible but, in practice, it's very difficult and approximately no freeform spells have been invented on our world that are both useful and reliable. Computation orbs in particular are necessary for spells with variable effects and many parameters; they compute the exact way the spell should be cast in each particular moment, for spells whose equations humans can't solve at anywhere near realtime. I thought your work up-front when 'preparing' spells might be similar, but since you can also cast some spells at will I'm not sure about that anymore."

"For example, applying force to an object is relatively easy, and some industrial applications that need the same amount of force applied at the same location every time can employ a not very skilled mage and use a simple and cheap casting implement. But flight requires applying different forces to different parts of the body, which change position relative to each other and are sized differently in different people anyway, and then also accounting for wind and altitude. And flying in combat requires very rapid and precise changes in orientation and separately the thrust vector, while moving at least the arms in order to shoot at targets; it would not be possible without modern computation orbs."

"Huh. Our spells are mathematically complicated objects that have to go through a preparatory process between the format in which they're written and the one in which they're hung and then change again when they're cast, but they're mathematically complicated objects that treat a lot of things as fundamental, like 'a person'. Fly is a third circle spell but I don't have it. - we should check if you can apply your flying force thing to a Floating Disk, because I do have that. They're not usually good to ride because they really want to follow you but if you cast one in an enclosed space you can sit on one."

"I can apply force to anything physical. It's ordinary force, the same as if you pushed it. Can your floating disk that you can sit on be pushed or lifted normally? I can make a magical shield that won't let physical objects through, if you push on it it absorbs the energy but if you apply enough force it breaks. A lot more force than pushing on it, of course. Maybe your disc is more like that."

"And I don't understand how it's possible to treat 'a person' as fundamental; the spell interacts with their body, it's not as if I'm somehow targeting the soul or their own magic or something... Maybe there's a complicated spell sub-unit that, uh, defines 'a person' and then various spells reuse it? But I'm not a magic researcher and I can't claim to understand how the spells I use actually work, I only know how to operate them."

"Not very normally, which is why it'd bear testing. It is a force effect, though, so maybe this won't work at all. Clearly your spells and ours are different in a lot of ways and the fact that ours can treat 'a person' as a natural thing is one of them, might be working on souls or not, I'd need to look into edge cases to form a confident guess about that."

If it operates on souls can it be used to determine how complex an animal needs to be before it has a soul? ...Tanya does not say, because she does not particularly want to get deep into discussions of souls, even though she's the only person on her planet remotely qualified to have one.

"It's worth testing. What other interactions might be useful? I can do various optical effects including lenses for zooming in on remote objects or for focusing optical spells, and mirrors. Visual illusions, which are broadly the same class of thing. Magical decoys, which imperfectly mimic my own mana signature - or someone else's, if I practice - and can be moved around me, this can help mislead enemies who target sources of magic like aerial mages do when they can't see."

"I can, in fact, obscure vision by creating a very small explosion with a lot of smoke. ...this uses up ammunition of which I have a limited amount, so I should be very conservative with it until I can confirm whether it's possible to manufacture more, presumably somewhere on the surface. It's the same ammunition I'd use for either penetrating thick metal armor, which I hope monsters don't wear, or for creating very powerful explosions which would almost certainly collapse a tunnel or something. So for offense I'll try to stick to light and heat, which don't require ammunition."

"Other utility spells... magnetic compass, precise rangefinder, using the illusions to try to hide myself - I can do that for you too, but I'm much more used to illusioning a patch of empty sky and I'll need to practice in this environment." Radio. Barriers and shields, which she can't apply to Belmarniss. A few other odds and ends.

"My spells usually have a range limit that has nothing to do with how well I can see, though some of them like the ray of frost do require aim and I guess optical tricks could help with that... Prestidigitation will turn things colors, and make little fragile things like the puppet show stuff, but there isn't a good visual illusion cantrip, they're all higher circle than that. I don't know if there's any critters that target by anything like a magical signature, I'd expect the ones that can't see in the dark to instead echolocate or sense vibrations or something. Most monsters do not wear manufactured armor but they can have scales that are about as good. I don't know what a magnetic compass is?"

"It points due north. Magnetic north, which isn't quite the same as geographical north, and I just realized I don't know where the north magnetic pole is on this planet, but it at least points in a consistent direction, which can help with not getting lost. ...no, wait, if there are magnetic ores or metals nearby it could get confused. Never mind, it's probably best not to rely on it underground."

"How can creatures that can't see in the dark live in the dark at all, do they all have ways of producing light? Or do you mean they're just blind? Flying obviously reduces noise and vibrations but I'll need light so I can see potential ambushers - I can illusion more light sources so they won't know which one I am but they'll definitely know someone's there."

"What determines the effective range of your spells? If the spell throws liquid acid it makes sense that it can't fly very far, but I'm not sure what the ray of frost is doing."