The Sharingan is a special physical condition that, passed down by birth, appears in only a few members of the Uchiha Clan. It is one of a select few types of pupil characteristic of certain Advanced Bloodlines. They say that the Uchiha Clan's originals lie within the Hyuga clan. Having at some point evolved the Sharingan from their Byakugan genetic trait. The Byakugan is one of the Advanced Bloodline pupil abilities passed down by birth to some members of the Hyuga house. It's an eye ability similar to the Sharingan but in some ways it surpasses the Sharingan. Neither of these special physical conditions are Jutsu, in that they cannot be learned and preformed by just anyone. These conditions are rather used in/with Jutsu to make them more effective.

The pupil of the Sharingan gives you the ability to read, and defeat all types of Tai, Gen, and NinJutsu. Making it possible to see a persons Chakra, in the instance of a Bunshin for example, the Sharingan will see it only as Chakra. With the same effect for all types of GenJustu. It also allows you to copy your opponents Jutsu once you've seen them, or even while they are being preformed.

Secret of the Sharingan, as told by Zabuza:
The Sharingan can neither read minds nor see the future. It is simply a trick that makes an opponent think you can. It's basically an eye technique involving hypnotism and an attention for detail. By using those two abilities together, you can progress from copying the mind to copying the Jutsu. This is the dance that makes it seem like a Sharingan user can see the future.

First, with that keen eye you copy your opponents movements and shake their mind.

• Body copying
  Once they are confused you are able to figure out what they'll say and can become them.
• Mind copying
  When their worry and confusion reaches it's peak, you attack with a clever trick. Through hypnotic GenJutsu, you suggest what seal they should preform.
• Jutsu copying
  Once you know what the seal they are performing, you simply copy it. Using this technique, Kakashi has gained the knowledge of hundreds, possible thousands, of Jutsus.

The Sharingan user's worst opponent would be a TaiJutsu specialist. When reading NinJutsu and GenJutsu, which need Chakra to be raised and Seal(In) to be preformed, you get an advantage through the Sharingan. Although TaiJutsu is a little different. Even if the Sharingan eyes can read movements, their body does not have the speed to react to TaiJutsu. Basically, even if a Sharingan user can see it, if their body can't move, then it's useless. Advanced Taijutsu is the worst possible match up for Sharingan.

Through the eyes of the Sharingan-user, Bunshins appear as flames of Chakra - the Sharingan sees through the illusion.
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