Why did Oliver Wendell Holmes, Jr. use the example of falsely shouting fire in a theater?

In the 1919 Supreme Court case Schenck v. United States, Oliver Wendell Holmes, Jr. used the analogy of falsely shouting "fire" in a crowded theater to illustrate the limits of freedom of speech under the First Amendment.

Holmes employed this example to explain that the right to free expression is not absolute. He argued that speech can be restricted when it is used in circumstances that create a "clear and present danger" of bringing about substantive evils that the government has a legitimate right to prevent. Just as falsely shouting "fire" in a crowded theater creates an immediate risk of panic and harm, speech that poses an imminent threat to public safety or national interests—such as obstructing military recruitment during wartime—may be prohibited by law.1