It's in the Halloween Special featuring stories such as the evil Krusty doll one, the one where Homer is King Kong. There is a story in there about Bart and Lisa, and in that, they bring zombies to life by accident. At the part where Bart accidentally turns Lisa into a snail using the book, she asks what he is staring at. He tells her that he never noticed what a nice looking person she is becoming or something. Lisa says, "Awe!" at what a nice thing he had said, yet her mouth does not even open!!!