Some instructional materials do not require getting permission or even evaluating the parameters for fair use. Materials copyrighted before 1923 (Twain, Shakespeare, Luther) are generally in the public domain. Just be careful about copying a recent version (expression) of a public domain work; that edition might be covered by copyright.
U.S. government publications are copyright free. Also, a growing number of researchers, educators and artists are sharing their work for free, a practice often called "open access".
Sites listed on this page offer video, text, curriculum and other open access resources that can be used without fear of copyright violation. Always give credit back to the original source, and be sure to honor any posted restrictions on types of use (for example, "non-commercial only"). See more at the Creative Commons website below.
Have you ever seen a website or document with a symbol like this?
The Creative Commons promotes content sharing and provides guidance for how to protect works while allowing others to use them for free. If you are interested in sharing your work with the world, take a look at the ways "CC" licenses can help you.