"22. The state has the power to protect its citizenry from actual harm, and thus has the power to outlaw one yelling “Fire!” in a crowded theatre. See, Schenck v. United States, 249 U.S. 47 (1919). However, yelling “Fuck!” in a crowded theatre does not create a clear and present danger to anyone and thus cannot be outlawed. Although they are both four letter words that start with F, the distinction is constitutionally significant."

Source: legaljuice.com