For an answer we need to look to what's often called the Doctrine of Fair Use, part of the copyright regulations that deal with the use of another's content without permission.
The U.S. Copyright Office lists a number of purposes for with use of another's content may be considered "fair," including criticism, editorial comment, reporting, teaching, and research. More important, though, are the four factors that determine whether a piece of content can be considered fair use.
- The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes
- The nature of the copyrighted work
- The amount and substantiality of the portion used in relation to the copyrighted work as a whole
- The effect of the use upon the potential market for, or value of, the copyrighted work.
When you look at these factors, copied directly from the Copyright Office, you can see how seemingly vague fair use is.
For instance, is there a certain number of words or paragraphs that can be used freely?
No? Not a limit of some kind?
So you can use as much as you want?
But you can use some.
Well, does it matter whether the content you want to use comes from a textbook or, say, a research book?
Um, it may matter. Or it may not.
That's not terribly helpful, Mr. Smartypants.
Maybe not but here's the key. Assuming you have a conscience, you should feel free to pick up and use a "little bit" of someone else's content. But the moment you begin to question yourself, gee, is this too much? Yes, it probably is.