Sheldon: The reason you're fixated on a good-natured simpleton like Penny is that she's the exact opposite of your first romantic attachment, your brilliant yet intimidating mother.
Leonard: Where on earth did you get that from?
Sheldon: It's in her book, "Needy Baby, Greedy Baby".
Leonard: That doesn't make it true.
Sheldon: It's called non-fiction for a reason, Leonard.