What makes a book a classic? (Your chance to weigh in!)

It is F. Scott Fitzgerald's birthday in a few weeks and it got me thinking: What makes a writer timeless?  What makes a book a classic?  Why do some books (like Gone with the Wind, Of Mice and Men, A Tale of Two Cities, Heart of Darkness) last and others quickly fade from the public consciousness?

And what defines a classic?  Who determines which books are classics and which are fads?  And what criteria do they use?

Furthermore, when can a book be determined a classic?  Is, for instance, the Harry Potter series classic?  Do we have to wait a generation or two before we can award a book that label?  Does it have to stand some test of time?  Or define some time period or era?

What say you?  This is your chance to weigh in: What makes a book a classic?


(Did you realize this post is entirely comprised of questions?  Isn't that fun?)