Good - But Is It A Classic??

Is it a classic?.... or just a really fun movie?

This morning, we got to talking about "Grease" with John Travolta and Olivia Newton-John. It came out in 1978 and my friends and I all saw it about ten times. Viewer Amy wrote in and said, how could it be a classic if she's never seen it?-- and started off a firestorm of emails!

What do you think? Is "Grease" a classic? How do you define a "classic" movie?

Marianne