I had this conversation with a fifth grade student.
Student: "Mrs. Q. ... Can I check out a book the library doesn't own?"
Me: (Going for simple logic) "If we don't own it you wouldn't be able to check it out."
Student: "No! You don't understand...I want to check out a book you don't have."
Me: (Realizing that I am not going to get anywhere with this conversation) "It's not here."
Student: "Okay, can I check it out next time?"
Me: (sigh) " ... Sure."