Encourage students to offer comments that are related to the question you're addressing. Try not to reward the student who says irrelevant things by giving them too much attention. In response to the example from the introduction, there are various approaches you could try. You might use some humor to refocus the conversation (i.e. "As interesting as dog food is to us all, we need to address the issue at hand"). Or you might try a firmer approach (i.e. "I'm not sure how these responses relate to the question I'm asking"). Try not to encourage irrelevant comments with responses like, "Why do you think you like dogs more than horses?" You may have a lively discussion, but it won't necessarily improve students' writing.