Fair use of copyrighted works is a limitation on the exclusive rights of copyright holders and is not an infringement on copyright law. Fair use helps preserve First Amendment rights of free speech and promote conversation for purposes such as "criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" (Section 107, Copyright Law of the United States).
If you use an existing copyrighted work in completely new or unexpected ways, your use may be considered as transformative and thus fair use, even when all four of the factors would traditionally weigh against fair use.
Examples of transformative use include:
To determine whether your use is transformative, ask yourself these three questions (adapted from © 2014 Kevin Smith & Lisa Macklin (CC BY-SA 4.0):