All kinds of things!
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Books
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Articles (Newspaper, Scholarly, Magazine, etc.)
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Websites
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Blog posts
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Tweets
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Movies
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YouTube videos
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Interviews
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Speeches
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Advertisements
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Output of a program that utilizes artificial intelligence
- ...and more
...essentially, anything you did not create yourself, that is not your own original thought, should be cited.
Also, did you know that you can self-plagiarize? If you quote your original thought as documented in another paper, for example, you should cite yourself!