Sharingan

The Sharingan (写輪眼; Literally meaning "Copy Wheel Eye", Meaning (Viz) "Mirror Wheel Eye") is a dōjutsu kekkei genkai, which appears in some members of the Uchiha clan. It is one of the Three Great Dōjutsu (三大瞳術, San Daidōjutsu; Literally meaning "Three Great Eye Techniques"), along with the Byakugan and the Rinnegan. The Sharingan is also called "Heaven's Eye" (天眼, Tengan), because of the many abilities it grants the user. Tobirama Senju defines it as the "eye that reflects feelings".

Abilities
The Sharingan's abilities consist of two parts: the "Eye of Insight" (洞察眼, Dōsatsugan) and the "Eye of Hypnotism" (催眠眼, Saimingan). The first of the Sharingan's powers is being able to see chakra flow. The Sharingan itself gives colour to chakra, allowing the wielder to differentiate them. They are also able to tell if a person is under a genjutsu because the person's chakra flow would be irregular. The Sharingan's second-most prominent ability grants the user an incredible clarity of perception, allowing them to easily recognize genjutsu and different forms of chakra, though not to the same extent as the Byakugan. This also allows the user to pick up on subtle details, enabling them to read lip movements or mimic something like pencil movements. As the Sharingan evolves, gaining more tomoe, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user, allowing them to see the image of an attacker's next move from the slightest muscle tension in their body and counter-attack or dodge without any wasted movement. However, even though the user may be able to see an attack or know it is coming, their body may not always have time to react.

The Sharingan's third and most well-known ability is that it grants the user the ability to copy almost any technique that he or she witnesses, apart from other kekkei genkai techniques. The user can memorise ninjutsu, genjutsu, and taijutsu with near-perfect accuracy, allowing the user to use the techniques as their own, or even modify them to create their own new techniques, much like Sasuke Uchiha's Lion Combo. In order to reproduce a copied technique, however, one must have the necessary skill or ability to perform them. A prime example is Rock Lee's brand of taijutsu. While Sasuke was able to copy some of Lee's moves, they put a much more significant strain on him since he hadn't trained nearly as much as Lee had. In addition to physical skill and kekkei genkai techniques, the Sharingan cannot reproduce techniques dependent on other factors, such as summons that the user has not signed a contract with.

The Sharingan's final commonly-used ability is capable of inducing a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent through genjutsu cast by simple eye contact between both parties. More advanced Sharingan users can take the hypnosis ability even further to the point where a powerful summon like Manda or even a tailed beast may be controlled. When this was used against Manda and the Nine-Tails, their eyes took on the tomoe pattern of the users' Sharingan while under hypnotic influence.

The hypnotic ability coupled with the other aspects of the Sharingan can make the user appear as if they can "see the future". First, the user must perfectly mimic every move their opponent makes and possibly their thoughts and words through careful observation. Then, they suggest an action to the opponent such as hand seals for a technique that the opponent and Sharingan user already know and copy that. With these things done, the user can complete the action even before the opponent does, possibly using the Sharingan's genjutsu to confuse or delay them. As with other genjutsu, genjutsu cast by the Sharingan are capable of putting an opponent to sleep, inducing paralysis, replaying memories to someone else via genjutsu, or creating various illusions to deceive someone. The potency of the Sharingan's illusion is dependent on the user's knowledge and skill with implementing genjutsu. Two Sharingan users are also able to free each other from other forms of genjutsu by first using genjutsu on each other, allowing them to enter their comrades mind and free each other from the genjutsu.

In addition, more rarely used by the Uchiha clan, the Sharingan can allow the user to perform two very powerful and unique genjutsu: Izanagi and Izanami, which alters or decide one's destiny, respectively. Using Izanagi, the user can warp reality for a short amount of time, changing reality into illusion and illusion into reality, thus escaping death and rewriting destiny; although to make use of this technique to its full extent, the user must have the DNA of both the Uchiha and Senju. Izanami, on the other hand, is a genjutsu that traps the victim in an infinite loop disregarding their five senses; this technique was specifically created to counter anyone who would abuse Izanagi's ability to alter their destiny, by forcing the victims to relive the situation over and over until they accept it and cease using Izanagi. Both techniques have been labelled kinjutsu, because use of either causes the eye that performed Izanagi or Izanami to close forever.

Although the Sharingan consumes the user's chakra while active, the amount of chakra that the user must exert is so insignificant that they can keep it active constantly. In fact, Itachi Uchiha was so masterful with the Sharingan that he was able to keep it active for most of the time, only once deactivating it after using Tsukuyomi on Sasuke and Kakashi, as well as Amaterasu to escape Jiraiya, due to exhaustion. However, when a Sharingan is transplanted to a non-Uchiha, the chakra drain is significant, leading them to cover it up when not in use. This seems to be circumvented by acquiring Senju cells in order to boost their vitality.

As mentioned above, the Sharingan, and particularly its evolved form, the Mangekyō Sharingan, can be used to subjugate the Nine-Tails. It has been stated in Uchiha lore and by Madara Uchiha who noted that the beasts were slaves to those with blessed eyes. Some wielders of the dōjutsu can also enter the subconscious location within a jinchūriki to directly suppress their tailed beast's chakra, as Sasuke Uchiha demonstrated when he entered Naruto Uzumaki's subconscious and suppressed the Nine-Tails' chakra.

Abilities
The Sharingan's power, because of its ties to the Curse of Hatred, is both a blessing and a curse. The moment that an Uchiha comes to know a powerful emotion towards someone, such as love and friendship, a stressful or emotional condition brought out from losing that precious person causes the brain to release a special form of chakra that affects the optic nerves while transforming the eyes into Sharingan. After its initial activation, the Sharingan may be used freely, although it is possible for an Uchiha to lose memory of such an event and not reactivate it until through willpower years later. When the Sharingan is first activated by a user, it has a different number of tomoe (巴) around the central pupil, varying between users.

The wielder of the Sharingan, through continued use and training, changes its form further, which involves the eye maturing fully until it has three tomoe. As the Sharingan develops, the user gains heightened abilities through the heightened ocular sense. For example, they can at least copy hand movements with one tomoe, predict a person's movement and copy techniques with two and completely use the Sharingan's abilities when it is fully matured. From what characters have noted, this dōjutsu apparently "glows" in the dark. True to its name, the Sharingan and its derived forms have shown the ability to spin. It is shown that when a person awakens their Mangekyō Sharingan without a fully matured Sharingan, their Sharingan will automatically fully mature while developing into a Mangekyō Sharingan.