Not that I'm trying to get into any of this argument, (I personally think using it once constitutes fair use, but Carlton, you've repeatedly used it)...

From the copyright office:

"The distinction between “fair use” and infringement may be unclear and not easily defined. There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission.

The 1961 Report of the Register of Copyrights on the General Revision of the U.S. Copyright Law cites examples of activities that courts have regarded as fair use: “quotation of excerpts in a review or criticism for purposes of illustration or comment; quotation of short passages in a scholarly or technical work, for illustration or clarification of the author’s observations; use in a parody of some of the content of the work parodied; summary of an address or article, with brief quotations, in a news report; reproduction by a library of a portion of a work to replace part of a damaged copy; reproduction by a teacher or student of a small part of a work to illustrate a lesson; reproduction of a work in legislative or judicial proceedings or reports; incidental and fortuitous reproduction, in a newsreel or broadcast, of a work located in the scene of an event being reported.”

Copyright protects the particular way an author has expressed himself; it does not extend to any ideas, systems, or factual information conveyed in the work.

The safest course is always to get permission from the copyright owner before using copyrighted material. The Copyright Office cannot give this permission."