OWL is a GREAT site for additional questions you have about all things MLA!
Note: If you are using a screenshot of a tweet in your paper, you label it "Fig.1" (or 2, or 3, and so on) and then follow that with the whole works cited entry of that Tweet. if the Tweet itself is very long you can use the first 3-5 words of it, but the entire tweet would be in the works cited entry.
ZoteroBib helps you build a bibliography instantly from any computer or device, without creating an account or installing any software.
