Though applied automatically the moment a work becomes fixed in a tangible medium, there are two things you need to remember about copyright:
Works in the public domain offer the most flexibility for educators in the classroom setting. With no restrictions on use these types of works can be quoted extensively, used in their entirety, or serve as the foundation for new creative works.
The majority of works in the public domain are there because of age. Anything published in the United States before January 1st, 1923 is in the public domain. Unpublished works of authors who died before 1945 are also free of copyright restrictions. Due to copyright law extensions that have taken place, the1923 date for published works will remain fixed until 2019. For a more thorough evaluation of complications with public domain in the United States, see this article by Peter Hirtle.
However, age is not the only factor. Generally speaking, a work can enter into the public domain in four ways:
*The work must be prepared by an officer or employee of the United States Government as part of their official duties. This means that works created by contractors may be subject to copyright restrictions, even if the work was federally funded. For more information on this topic, see this article by Bonnie Klein.