Plagiarism

I have been writing for quite some time and I have never even considered that I might be plagiarizing, yet there have been moments in my college writing career when it was so heavily focused on in the discussion of the research paper that I felt that I might be guilty. It seems strange to me that there can be so much problem with plagiarism, but maybe that has to do with the definition. I have always thought that plagiarism was intentional, word-for-word copying of someone's thoughts and ideas. I think that teacher's hold the key to distinguishing between plagiarism and misuse of sources or faulty citations. When teachers overemphasize the aspect of plagiarism that I have labeled as poor research, I think student's feel the same ridiculous guilt that I have felt during these ineffective discussions.
I have never given much thought to common knowledge, but I guess that it usually seems to reveal itself as I am doing research. While I may cite a certain fact during my early research, if I subsequently read it in four or five other sources, I tend to incorporate it into my own knowledge base about the subject. I think this is another place where teachers have to really focus on the research process; if these things are solidly understood, I think many of the "plagiarism" issues will work themselves out.