Watch Welcome to the Hellmouth (season 1 episode 1) when Buffy goes back to the library and she and Giles are arguing about her being the slayer. Giles hands her some books (Buffy asks him if he signed up for the time life series) when he hands them to her the spines of the books are facing one way when she gives them back to him they are facing the other way. They alternate like that a few times before they are finally put down.