"Are we allowed to use ideas from our writing exercise to help us write this paper?" she asked.
"Of course," I said. "That was the whole point of the writing exercise—to get you a head start in thinking about how you want to approach your paper."
"OK," she said. And then after a brief pause: "Because at orientation they told us we weren't allowed to use our own work twice."
"Ah," I said. "That doesn't really apply in this case. And anyway, I don't really mind, in this course, if you take a paper that you've written for another course and revise it for an assignment in here. You just have to make sure that what you turn in fulfills my specific assignment. Other professors might feel differently, though. So I would always ask before you tried to do that."