Information in the public domain is information that you can assume is general knowledge, such as the fact that the earth moves around the sun or that cheese is made from milk. These facts are so commonly known that you do not have to document where you got them. However, if you were to state facts regarding new discoveries about the composition of soil on Mars or new methods of releasing condors into the wild, you would, of course, need to document your sources.
Famous sayings are often in the public domain. For example, if you refer to the quote "To be or not to be," you can safely assume that your readers know the quote came from Shakespeare. However, if you are quoting a recent speech by the president of the United States, you would need to cite your source in your research paper or writing assignment, because your audience may not have heard the speech and may not recognize that your quotes or information came from the president.
A parrot's ability to talk and repeat words is common knowledge; you may assume your audience knows parrots, and there are many sources from which the information could have come. But if you were to refer to Dr. Irene Pepperberg and her recent studies with Alex the parrot, you would have to cite your source.
If you happen to plagiarize information, original writing, or ideas — either accidentally or on purpose - you're likely to end up losing more than you thought you'd gain from the literary theft. As students have come in contact with term paper sales on the Web, teachers worldwide have begun to realize the need to be able to spot the unfortunate use of intentional plagiarism. A Web site called Plagiarism.org allows teachers to submit students' papers for analysis of plagiarism by comparing term papers with thousands of archives from online cheating sites and with papers from past semesters and other universities.
Remember, when in doubt, cite your sources.