What makes a book a classic? Does it have to have been published before a certain date or have reached a certain level of popularity? If a book is written by a certain author, e.g. Charles Dickens, does that automatically make is a classic? What are the requirements?
I don't know whether there is a specific formula that makes a classic, but I think they should be books that everyone has heard of or are familiar with to some degree, regardless of whether or not they have read the book. I think classics should also always be suitable material for study in the classroom, whether that be secondary school, college, or university. I used to think a classic had to be literary, but don't think that is the case, because The Lord of the Rings is genre fiction, and that is a classic. Classics don't have to be old either. You can have modern day classics, and Harry Potter is probably one of them.
One where the themes are still relevant, and haven't become obsolete. It usually needs to be well written (though not always). However, a lot of the time, I think people just say it's a classic because they've been told that at school or everyone thinks it.