Because even though you have substituted some of the words with your own, the original idea belongs to someone else, and you haven't documented your source. For help with correct citation formats and other rules of documentation, see Citing Sources . There is a common misconception that only printed sources of information, like books and magazine articles, need to be formally cited. As a general rule, to avoid copyright infringement you must obtain permission before posting the material. Copyright and the Internet.
Common knowledge is something that is widely known.
For example, it is common knowledge that George Bush was president from 1989-1992. It would not be necessary to cite a source for this fact. The source of this information would have to be cited. If you're not careful while taking notes, it may be hard to tell later on whether certain passages were copied exactly, paraphrased, or simply your own ideas that you jotted down as you were reading. Sloppy note-taking increases the risk that you will unintentionally plagiarize.