Students and plagiarism

One of my tasks as an information literacy teacher is to discuss the ethical use of information with students. To put it simply it's a question of using reference technique correctly and being aware of the difference between your text and somebody else's text. So if you refer to other authors in your text in a correct way and the author's name combined with the article she has published is found in the reference list you should feel secure. But then there is of course the discussion of when something becomes common sense, common talk and doesn't need to be referred to? I had a student coming up to me after a lecture about reference technique and he was really worried about how he should know which thoughts are his own and which are not. And he added that he had been reading a lot since he was three years old...Can he ever be sure that he isn't plagiarising someone somewhere? My answer was that if you feel unsecure you can always use your information searching skills to try to find out if there's someone that has come up with the same idea. But being afraid of plagiarising shouldn't cause fear of writing. It's easy to find traces of plagiarism if someone has copied and pasted a text but how should we find the owner of ideas? I suppose the experts in immaterial rights can answer this...Anyway I am afraid that the plagiarism talk today, plagiarism discourse could even result in the death of creativity and passionate writing.