 One concept that can be confusing when deciding whether information needs to be cited is that of common knowledge. Common knowledge, simply put, is information that is so commonly known that when you include it in a piece of writing, it does not need to be cited. But how to know if something is common knowledge? One way to decide is to ask these two simple questions. Is the information factual and not controversial? And, is it widely known among your readers? For example, if I state in a paper, George Washington was the first president of the United States of America. This is a fact that isn't controversial, and it is also very widely known. You might consider this piece of information common knowledge and choose not to cite it in your paper. As a second example, what if I state, the gravitational pull of the moon as it orbits the Earth affects the ocean tides. It is factual and not controversial. But is it widely known and accepted among your readers? If you are writing a paper for your astronomy class where you've discussed or read about this fact in your textbook, you might consider it common knowledge among your class peers. When in doubt, ask your instructor if you should consider a piece of information common knowledge. Let's consider a third statement. A vegetarian diet is healthier than eating meat. First, is this statement a fact? Although you might argue that many people agree with it, this does not mean it is a fact, and it does not mean that there isn't debate about whether it's true. If you make a statement like this in your own writing, make it clear that this is your opinion, and then back it up with evidence. If it is someone else's opinion, make that clear and cite it according to your citation style guide. One final note on a frequent misunderstanding about common knowledge. If a work is free to access, or if there isn't a specific author as in crowdsourced sites like Wikipedia, this does not mean that it's common knowledge. These are two completely different concepts. Information found on a site like Wikipedia or other free websites still needs to be cited. Now, whether Wikipedia should be used as a source in an academic paper? That's a question for another video.