Fair use is the flexible and dynamic exception to copyright law that serves to balance the rights of creators with the public interest in using copyrighted works to advance education, to comment and criticize, and to make new creative content. Its flexibility is often somewhat intimidating or frightening because when considering fair use it is very rare to know with certainty that a use is fair, only that it is more or less likely to be fair. It can be frustrating that the law does not give us any clear answers regarding amounts we can use and know that we are "safe". Fortunately, there are ways of understanding the purpose and function of fair use that can help us feel more confident about evaluations and maybe even come to love fair use for its flexibility.
It is important to remember that using a checklist will never let you determine with certainty whether or not a use is fair. It can, however, help you organize and, importantly, document your thinking. It's a great idea to use and print a fair use checklist whenever you are dealing with a tricky fair use situation. Take the time to document your thinking in case of a future challenge!