What is teacher's role if he caught a student while involved in plagiarism?

Most of the time students don’t understand what they have plagiarized. I would simply question them on what they have written, and go accordingly.
I remember watching my son find a paper online, then use a thesaurus to change all the words, thinking he had created his own work. I can’t imagine the teachers weren’t wise to that.

I think if the student is very young, elementary age, the teacher’s role is to explain why it is plagiarism, why it is unnacceptable and teach them how to write for themselves or give credit to the orginal author. By high school students are either graded down for plagiarism or even possibly risk as a harsh a punishment as expulsion if it is seen as actual, purposeful, cheating.

It is difficult for younger people to reword concepts and draw their own conclusions from multiple sources. A child can make all the right efforts to research and learn about a subject and still wind up plagiarizing without really understanding what they did wrong or how to overcome it. They should get credit for the work they did to learn about the topic and then given guidance on how to process the information and writing skills.