Let the professor go first

You wait for him to speak; no sense implicating yourself unnecessarily, after all.

"Bryan, I have a favor to ask, and it won't affect your grade if you say no."

Before you realize what you are doing, the grin has already spread across your face. The last time a teacher asked you for a favor, it ended with you shooting a wad of semen all over the term papers of your tenth grade classmates in Mrs. Keyes' Social Studies class. You got an A for the class, and your classmates got an excuse that Mrs. Keyes spilled some egg whites on their papers because she was grading them while cooking dinner. She was cooking, all right, but not in the kitchen!

Then remembering yourself, you erase the smile from your face and choose an appropriate response.