Sander immediately sat up when the Professor appeared. Then the next thing he did was to look down at the cookie he had been chewing. It wasn't a cookie?

He listened as the other students gave their answers. So this place had been enchanted after all. Thinking of a possible answer, Sander raised his hand, "Professor, could it be a charm that makes you think you're seeing something, but which in fact, you're not?"

Sander didn't really know the name of such a spell so left that part out.