Copyright is a funny thing, as one day it just runs out and leaves works created decades ago out in the public domain.
Every year, on January 1, certain copyrighted works lose what is called their "protected status" and enter the Public Domain, meaning anyone can use such works as raw material for their own creations (without the worry of facing a lawsuit for doing so).
Famous examples of work in the Public Domain, which have been remade and retold on countless occasions are the works of both William Shakespeare and Jane Austen - their expired copyright is the reason why we've witnessed a zombie version of Pride and Prejudice, Star Trek Klingons reciting Hamlet and an abundance of films based on stories told by these icons of English literature.
