In order to balance the interests of the creators of copyrighted works with the public's ability to benefit from those works, copyright law includes the exemption of Fair Use.
Fair use allows limited use of copyrighted material without permission for purposes such as criticism, parody, news reporting, research and scholarship, and teaching.
However, just because a use is educational does not mean that it automatically qualifies as fair use. When using content for certain purposes it is important to consider copyright and if you might be infringing on it.
Fair Use Evaluator - American Library Association
Stanford Copyright & Fair Use Center - Stanford University Libraries
Reproduction of Copyrighted Works by Educators and Librarians - U.S. Copyright Office
For more information on this topic see the Fair Use & Fair Use Analysis Tool pages on the UNM Copyright Guide.
Works that are in the public domain in the U.S. are not protected by copyright because
You can use any U.S. work in the public domain in any way that you want, as much as you want. They belong to the public. Keep in mind, however, that the requirements for public domain vary by country.
There are several great resources for finding and accessing materials in the public domain:
Depending upon when a work was created, it is subject to different requirements regarding copyright notice and registration, as well as different copyright terms.
Copyright Term and the Public Domain, a guide to copyright duration created by Peter Hirtle at Cornell University, is a comprehensive and useful resource for researching a work's copyright status. You can also use the Copyright Slider from the American Library Association for quick reference.
As a general rule, works registered or published in the U.S. before 1923 are in the public domain.