Training with Genma at Jusenkyo in China, Ranma was thrown into a spring called Nyanniichuan. He emerged as a young girl, according with the spring's curse. When splashed with hot water, he changes back to a boy, but when splashed with cold to room temperature water, he turns into a girl again. As for our well-built 16-year-old hero/heroine, the martial arts is his life. He also possesses a rather large and easily wounded ego, believing his physical strength to be his only virtue. Whenever his pride is damaged, Ranma feels the need to rectify the situation, even when it's not necessarily in his best interest to do so.

(Source: Wikipedia)