"The Amount and Substantiality of the Portion Taken
The less you take, the more likely that your copying will be excused as a fair use. However, even if you take a small portion of a work, your copying will not be a fair use if the portion taken is the “heart” of the work. In other words, you are more likely to run into problems if you take the most memorable aspect of a work. For example, it would probably not be a fair use to copy the opening guitar riff and the words “I can’t get no satisfaction” from the song “Satisfaction.” "