Students are more likely to participate when they see how activities relate to their own writing. For this reason, explain to students how an activity will help them become better writers for the next assignment.
For example: "To write effectively, we have to consider the context of our audience. This will help focus our writing so that it speaks to someone with different expectations. Since the context for essay three is not a familiar academic situation, you'll need to analyze your context and audience before constructing your argument. This next activity is aimed at helping you think more about the context for which we'll be writing."
