Any time the original thought, words, or pictures did not come from your own head, you must give credit to the original source by showing exactly where you found the material. This is called citing, or documenting, your sources. This is important. Avoiding plagiarism is not an exercise in inventive paraphrasing. There is no magic number of words that you can add or change to make a passage your own.
Original work demands original thought and organization of thoughts.
But the person whose work is stolen is not the only one who loses. People who plagiarize also lose, because they miss out on the learning that occurs in the process of conducting research, reading, writing and revising. Breaking these rules can result in a failing grade for the course, suspension, or even expulsion from the school. Paraphrasing is rewriting a passage in your own words. It is important to remember that even if you rewrite a passage, you must still cite the original source of the idea.