Some would argue that it isn't because they simply put the idea into their own words, henceforth it is not stealing. This is just the basic definition, and there are many more tangents that go along with this basic concept. Plagiarism is not just cut and copying someone else's work and passing it off as your own, using someone else's idea is also considered plagiarism. If you were once unfamiliar with a topic and decided to write about it then basically you need to cite everything you type because none of it is your own. It then goes on to say that common knowledge is excluded from being cited.
Common knowledge is such a broad term, and can mean something totally different to everyone.
Something that I consider common knowledge, would not be considered common at all to another. This is where students fall into the trap plagiarism. Passing someone else's idea off as your own is also considered plagiarism, even if it is reworded so it is not a direct quote. This is also another trap that students fall into easily. Most of my information on plagiarism came from Dr. Humphries when she gave my class a presentation and explained to us what needs and doesn't need to have quotes, footnotes, or sources.