Nominally Ninja

There are a lot of things about the Sharingan out there. How and why it evolves is all sorts of in debate, what exactly it encompasses, where it goes from Uchiha to the bloodline of another clan. And, above all that, how it's assembled.

The Sharingan, contrary to what some people might believe given the appearance doesn't spin 360 degrees in the eye socket. The eye had moving parts, unlike other doujutsu which are just on and off without anything spinning about. The appearance is actually the result of a separate layer on which the tomoe rests above the iris which activation of the Sharingan the first time develops stem cells into. When this happens naturally, the eye has time to adjust and heal and is stable given that this layer is designed to protect the eye from chakra damage, and done correctly will activate each subsequent tomoe as they connect to the appropriate chakra pathways in the brain to activate previously unused nerves deep in the eye. This also accounts for the blood-red color, as the detachment allows a very small amount of blood to fill the gap under the tomoe over the iris, obscuring the natural color and making the tomoe appear black due to the application of blood and chakra in the area. This in and of itself is not dangerous for a Sharingan user as there is a set pathway where the tomoe to spin over the iris to keep damage from direct chakra application from occurring.

The pathway the tomoe travel is incredibly important to how the Sharingan itself works, given each tomoe's addition along this path gives a different sort of perception, be it a seeming slowing of time or awareness of chakra to the visual spectrum that is only usable when chakra is being fed to the eyes. When the chakra is retracted the blood also returns to the inside of the eye, allowing the tomoe to return to their natural position and their seemingly black color. Since the tomoe are naturally the same color, being made of the same iris themselves originally, they are not visible in this state. Of course, when one doesn't have dark eyes, it's far more obvious, though such Uchiha are incredibly rare.

Of course, when it's said that this is what happens when the Sharingan happens naturally, this means when the Sharingan is given time to essentially manifest on its own and the person wielding it doesn't have to experience maturity on the fly as their body tries to compensate for something it's not ready for. Using trauma or attempting to force an echo out of a child by showing your own Sharingan isn't appreciated by those in the clan who believe that these sorts of activations make the people they're inflicted on unstable. They're right in this belief of course, because if the body isn't ready, then the damage isn't to the eyes in this case, it's to the mind itself as chakra is forced through unprepared paths in the mind and change how the Sharingan user might perceive the world even when it's off. Unfortunately, war makes many children into victims of this sort of activation, and the lucky ones are the ones who manage to hold off until their bodies can keep up.

The Mangekyou Sharingan, which destroys the simple pathway that the basic tomoe rested in when they were initially released, is a different animal entirely. Due to how it's activated, and the shock of chakra that comes with the connection of this level of Sharingan, there is usually an immediate spread of damage. At times, the pupil itself is partially obscured with blood in the new pattern that forms from parts of the iris that shouldn't be involved, and during others, this is not the case, but in the stall of movement there often causes more damage to the eye in general. In a typical, emotionally driven chakra blow of an activation the body can't tell the difference between normal Sharingan and a Mangekyou. Thus, the eye spins as it normally does, but feeds in increasing amounts of chakra which starts a steady decline in vision as the eye's chakra pathways are burned from the outside in. Even as this is going on, it connects more nerves within the eye to heighten what can be done, thus increasing the abilities of the user even as normal vision starts to go. The body, unable to repair the damage, tries to compensate by flushing more blood into the eyes, which is why bloody tears after use start to occur when the damage is severe enough.

Ultimately, this leads to blindness. The solution to this kind of activation is switching eyes with someone else who is similarly afflicted, as the mutual damage will actually allow for sight to be stabilized, and even fixed back up to a higher level. This is called the Eternal Mangekyou. The body, on receiving a new set of eyes, will immediately try to connect to them and activate its stem cells again for a short time, allowing the damage to heal. In doing so, new connections will be forged, and the pathways that the body had accepted from its own, previous, Mangekyou, will be imprinted over the new eye in a controlled fashion as an attempt to repair it. Because the eye in question did not actually belong to the person who is healing the damage originally, this means that there will already be a pattern of damage, thus resulting in the phenomena where neither pattern survives the healing, and instead a new one is formed. This new pattern will be entirely stable, working much like the initial movement of the tomoe over the iris that the first stage of the Sharingan has, allowing a free spin without chakra being burned into the pathways of the eye. Unfortunately, replacing the eyes is not enough to fix damage previously accrued to the mind by errant chakra due to the way the eyes were activated and used.

Of course, while this usage of the Mangekyou is the better known one, it is not, in fact, the only way to acquire one. Given how young most shinobi are when circumstances arise that would lead to a trauma-induced Mangekyou, it's forgivable that it might be assumed it is the only way. Yes, not everyone can mature into a Mangekyou, while most can activate it via trauma under the correct circumstances, even that isn't guaranteed. If it was, the loss of a teammate would universally be enough to activate the eyes, but that is not adequate in most cases or many many more Uchiha over the generations would have had these eyes. By the same token, among even this smaller number, there are those who come into it naturally. These Uchiha are often able to use the abilities unique to them before the eyes even visibly change, even if wholly unintentionally. Obito is an example of this kind of Uchiha and should have by rights been dead under the rock as presumed. Instead, he very likely dropped out from under the stones via Kamui without even realizing it as soon as he lost consciousness.

These Uchiha, because their eyes were not forced, are often older, and sometimes well into their twenties or thirties before their eyes manifest in this way, and will not turn blind due to the use of them. By the same token, this slow maturation that often progresses until an Uchiha stops growing, which can be as late as their early thirties, might even take a Sharingan wielder as far as having a Rinnegan. This, even rarer than the natural manifestation of a Mangekyou, almost never happens because of continued stress on the body due to shinobi life, and in using their eyes often and vigorously stunts the option in many. Madara was an example of going from a Trauma Eternal to a Natural Rinnegan, as he's seen with the ringed eyes before he ever left Konoha in multiple scenes, though there are no indications he was aware of this until much, much later in life, whereupon he presumes it was Hashirama's genetics added to his own that caused it.

While clearly obvious that the Uchiha have many different facets to their eye use, everyone has to circle back and go 'but what about Kakashi?'

They actually rather have a point. Kakashi shows that the eyes are not strictly attached to the Uchiha as a clan, but we see him essentially collapsing for a very long time any time he ever uses the eye he was given for more than a few minutes at a time. This is, in fact, part of why the Uchiha are not overly concerned about anyone stealing their eyes. People use them and die of the chakra drain. Alternately, they're given an eye that was off at the time of death (or thievery), and this, in turn, is permanently stuck on this setting because most people do not have the capacity to feed chakra to their eyes in any kind of directed manner. A Hyuuga given Sharingan, for instance, could probably make them go, but the stress would probably not be negligible. Also, the nature of the Hyuuga clan makes this incredibly unlikely even if they were not an allied clan after the villages were created.

Which brings us back to how Kakashi did survive, and the rather worrying implications of Danzo and his arm. Kakashi has a unique chakra. This is a fact, a point which is in no way in contention. The chakra is described as being heavy, dense and I suspect this means that a lot less of it goes a lot further, which is how he was able to be such a ninjutsu beast even with a perpetual drain that he didn't seal quiet but instead merely covered. Danzo, on the other hand, created special braces to seal down the chakra in his arm, both Senju and Sharingan, and kept the eye in his head not only covered, but sealed, lest any sensors in the village actually notice it. Certainly, his ability to fight and use a Sharingan specific ability over and over is impressive, but that might just be points toward the fact that the man is old and thus had a long time to build up that power. No, more concerning is that all these eyes were on. This, from what I can discern, means that the eyes were taken when they were on. From someone alive. If this someone was the original Uchiha or not is impossible to say as he might have simply had someone from the clan who was turning on quiet Sharingan form the massacre for him in ROOT. Alternately, he collected them over the years leading up to the massacre, and thus didn't actually get any of them then at all. It's impossible to say with any certainty, though it's an unfortunate truth that the eyes were almost definitely not retrieved painlessly.

Regardless of all of this, it's understandable that the Uchiha, as a clan, tends to make provisions for the eyes of their dead when they have them active, often bequeathing them to other members of the family as a 'just in case' sort of measure should anyone run into an event that might eventually lead them to blindness. It is something that wouldn't be shared with outsiders to the clan, because most clans simply don't go about giving their eyes to people like the Uchiha do as a matter of both tradition and survival. Even the Hyuuga would never consider such a thing, instead preferring that their members have them melted rather than shared. Understandably, the Uchiha find such a practice both wasteful and horrific, and would never ascribe to such a thing, which often leads to less than cordial relations between the two clans.

I keep throwing around the words Natural and Trauma and I'm sure some of you are wondering how one can even tell. First, Trauma Activation, is more or less what it says on the tin. For a first stage Sharingan, this is often the fear of loss, rage, or horror in an amoung well past what someone experienced before, which will leave the Uchiha in question in a state of emotional disquiet intense enough that it affects their chakra and pushes their eyes to form as a desperation response. Similarly, the reaction of intense grief and guilt most activate their Mangekyou with is emotional, painful, and both instances affect the mind, following hormone and chakra pathways through the mind and cutting these sensations deep and fresh. They will not ever be forgotten, they will not ever be fully gotten over. They are there to stay because the Sharingan always remembers. In doing so, in some ways the person is marked for life by these events, and even if they heal in most ways they will always perceive the world through the prospective filter of these feelings.

In sharp contrast, those who come by the Sharingan Naturally go through many many steps that those who activate it in the more traumatizing manner never do. Certainly, the eyes will come on eventually, but well before that is the mimicry. It's not intentional, it's often not even noticed by the person doing it, and those being mimicked, if not Uchiha, are often either flattered or angry by what they presume is a mockery. Imagine, if you will, a genin absently falling into the same walking pattern as his female teammate. Or perhaps an Academy student who takes to doing the exact same tap tap tap that the child at the desk in front of her often does when they're thinking. The examples go on endlessly, truthfully, but these sorts of things are simply part and parcel of Sharingan life for a young Uchiha. See and do, often personal ticks that people write off as being due to spending a lot of time around someone else when it's instead their eyes gearing up to notch up into copy-paste mode. A singular lack of grace might also be a side effect of this as their body might be trying to do one thing while their mind is trying to do another, so it's understandable that more training is often required in this sort of instance.

Eventually, a Natural Activation will simply happen, be this in combat or while sitting around at home reading a sappy novel. Leveling up usually doesn't happen until much later, though a Trauma induced activation of a Mangekyou following a Natural base activation gives a chance that the eyes won't actually degrade over time with use. This is not, sadly, any kind of guarantee, as if they were not predisposed to it in the first place damage might have happened either way. Similarly, a Rinnegan, even with all these other factors, is almost laughably uncommon, but if a Sharingan user simply doesn't use their eyes for a period of months or years, then the restive state might be enough to push them that far if they have the natural ability to do so without outside interference.

Which brings us to usage. Anyone who knows an ambitious young Uchiha knows an Uchiha who passed out in the library trying to read the whole thing with their Sharingan on. It's a ridiculously common problem because the vast majority of the clan happen to be frustrated nerds who want to learn all the things and happen to not always have the means to do so. In fact, it's known enough in Konoha that it was the first stop anyone made for a new Sharingan user once upon a time, and the librarians knew to keep an eye on young Uchiha to shove them out the door if they looked like they were about to faint of chakra exhaustion. Some misunderstandings outside of those in the know caused problems because of this, while those who knew just found it amusing, ultimately. Even Itachi fell prey to this particular mishap as a child.

Other uses of Sharingan are a bit more unconventional. There are, of course, the many Mangekyou abilities, which vary as wildly as the Uchiha that use them, though Susanoo is one of those abilities that seems to be embodied in every Sharingan pair of sufficient level. No, the big ones are Izanagi and Izanami, the paired jutsu, one of which changes reality in a genjutsu that changes time itself, and the other which traps someone inside their mind on a loop until they get the idea that the caster is going to make them change whether they like it or not. Both of these blind the eye used and neither requires anything more than the basic Sharingan. One need not be an Uchiha to use them either.

Of course, there are plenty of other things that the Sharingan can be used for, outside of combat, including things as simple as forgery or staring at someone in a moment you dearly want to remember. It's not all about jutsu, and has plenty of intelligence uses as well.

Finally, there's the mood ring thing. Uchiha, well, their eyes are attached to their emotions, though most people presume it's mostly an anger thing because they activate them most often in combat. Sometimes fear. No, there's also embarrassment, happiness, and the biggie. Arousal. Nothing like looking an Uchiha lady in the eyes after you get her all interested and thinking she's furious when really she just wants to jump you. In a nice way. That has led to many an interesting misunderstanding, at the very least.

Now that the eyes are all sorted, some of their clan dynamics will probably make more sense.