How many times have you heard a teacher or professor say this?
But many students might wonder what exactly counts as a fact, and what doesn't.
General knowledge, such as 'Crick and Watson discovered the structure of DNA,' will not need referencing.
Common knowledge in the field is generally fine, too, although you should err on the side of caution.
If you get an answer, it could be common knowledge!
A good rule of thumb for any writer is to go ahead and use a citation when you're not certain whether or not the citation is necessary.
For example, a psychologist will be aware of pavlovian conditioning, so you do not need to reference that if it from your own head. There are a number of ways in which you can reference the source, but most are based upon variations of MLA and APA style.
Check with your supervisor which exact technique you should be using, and be consistent.
For example, This makes it clear that you could not access the original work, and that you correctly attribute the original findings to the researcher who actually performed the initial research.
A few standards, such as Chicago style and the Council of Biology Editors (BCE) use a footnote numbering system, where a number is used and cross-referenced with the endnote section and bibliography: It is always best to over cite, and avoid accusations of plagiarism, but there are a few times that citation is not necessary.