Students at all levels often generate ideas that are not obviously right or wrong, and, when they do, it is productive to take time to explore the ideas as a group. Some of these ideas are really strange. Occasionally, they are so strange that even the teacher might not know what to think of them. This is the story of what Cosby's rule is, how it came to be, and the effect that it had on an AP Calculus class in a small school in rural Georgia.