But most don't. An unsuspicious discussion will often lead students to reveal unintentionally plagiarized sources and to request help for incorporating the material in an acceptable way. Post excerpts from the best work on the course web site, and congratulate those who did good work. Have students exchange papers or read their papers aloud to each other in small groups, and discuss each others' papers. Schedule conferences with students about their writing, and ask questions about the context of their ideas and more of what sources had to say.
Plagiarism is a moral offense, rather than a legal one.
Most instances of plagiarism are not violations of the copyright law, and most violations of copyright law are not plagiarism. The penalties for plagiarism can be severe, ranging from loss of respect to loss of degrees, tenure, or even employment. At all stages of research and writing, you must guard against the possibility of inadvertent plagiarism by keeping careful notes that distinguish between your own musings and thoughts and the material you gather from others.