Why didn't you just give me the answer?

My mom was trying to explain to my brother some mythical characters, after which a movie they were planning to watch, is called. She noticed in the middle of the process that she had forgotten what the creatures did and why they did it. "Hana, get me the encyclopedia", she said, and my brother laughed, because it has been ten years since he last touched the encyclopedia. I handed the book to my mom and she read the description of the mythical creatures out loud, while I made myself a sandwich. My brother sat there listening. After they were done, I murmured: "It's explained somewhere in the film."
"It's in the film?! So you knew what they are and that they are going to be mentioned in the film?!!" My brother looked at me with a somewhat reproachful question and an amused I-can't-believe-it laugh on his face.
I shrugged, smiled mischievously, and noticed: "I'm a teacher."