I have made it up to episode 5 of Umineko, and I've found one incident in particular unusually easy to resolve (easy enough that though the answer hasn't been suggested by anyone in-game, I am sure that I know how it was/could be done); I'm wondering how much it is due to specialized knowledge and whether it really looks harder to other people. (Because of the curse of knowledge, it's now difficult for me to see whether the puzzle really is as trivial as it looks to me.) So, a little poll, even though LWers are not the best people to ask.

In episode 5, an unknown caller phones Natsuhi in her locked personal room. He says he's predicted her favorite season of the year, and asks her what it really is. She replies 'winter', and he says that is what he predicted. She is skeptical and he tells her to look underneath a clock in her room. She does and finds a slip of paper with the word 'winter' on it: he had been there earlier and left it as proof of his prediction. Natsuhi is shocked and mystified.

How sure are you that you know how he did it?

How would you rate your familiarity with cryptography?

(Please rot13 any replies.)