You should cite your sources because otherwise you would be taking credit for ideas that were not your own, otherwise known as plagiarizing. If you don't cite, you are stealing someone else's intellectual property.
Many students who plagiarize do so unintentionally, simply because they are not aware that what they are doing is, in fact, plagiarism.
Unintentional or not, plagiarism does have its consequences, and those consequences are determined by SUNY Delhi's Academic Integrity Policy. Use Resnick Library's guide to help you understand the various types of plagiarism, and make sure that you are not an accidental plagiarist.
When you or someone else creates an original work, whether it be a research paper, a poem, a song, or a screenplay, that work is copyrighted (so long as it is in some tangible format, i.e. on paper, on a disk, etc.). To understand more about copyright protections under the law, try the following resources:
A copyrighted work is generally protected for 70 years after the death of its creator, so that only the creator (or the creator's family) can benefit or make money from his/her creation. If not for copyright protection, you could write a screenplay and submit it to a studio, and that studio could use your screenplay to produce a film without consulting or compensating you.
If something is common knowledge, then you do not need to cite it (unless you quote directly from a source). Say, for instance that you knew that Abraham Lincoln was the United States' 16th president before doing your research on him. The fact that Abraham Lincoln was our nation's 16th president is common knowledge. The fact that he was president from 1861-1865 is also common knowledge.
You may also consider factual information to be common knowledge if you've found it within 4 or 5 readily available sources.