The Sharingan is not automatically activated in the user by birth. It is only first unlocked under some stressful or emotional condition after which the Sharingan may be used freely. 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, evolves it further which involves gaining more tomoe, up to a maximum of three per eye. As the Sharingan evolves, the user gains heightened abilities.
First Tomoe- 500 words required
Second Tomoe- 700 words required
Third Tomoe- 1000 words required
The Mangekyō Sharingan is a heightened form of Sharingan. The Mangekyō Sharingan gives the user access to powerful and forbidden techniques. The Mangekyō Sharingan is distinguished from a normal Sharingan through its appearance, which changes the form of the tomoe seal. The Mangekyō Sharingan gives the user access to several powerful techniques, including the powerful fire jutsu Amaterasu and the almost unbreakable genjutsu Tsukuyomi. The Mangekyō Sharingan erodes the eyesight of its users, eventually causing blindness, which can be alleviated by obtaining another pair of Mangekyō Sharingan from a sibling.
Amaterasu- 2000 words required
Tsukuyomi- 2500 words required
Susanoo- 4000 words required