I read this in a magazine many years ago, but I like the snappy retort:
Q: "Are you pregnant?" A: "No, I'm just carrying it for a friend."
One of my coworkers once asked "What date is Christmas this year?"
The dedicated readers of Mills & Boon romantic fiction are a breed of their own. They used to come into the public library branches and only check the returned books which we left on the trolley near the circulation desk.
Silly question on seeing an empty trolley: "Aren't there any books in today? I'll come back when you get some in."
But my favourite is still: "Do you have the book called Tequila Mockingbird?"