The colon is a versatile punctuation mark. Here are its three primary functions, followed by a few other uses:
1. Definition or expansion
“But here’s the interesting thing: He hadn’t ever been there before.”
Note the capitalization of the first word after the colon. All usage guides agree that in a sentence like “I want you to tell me one thing: the truth,” the first word should be lowercase because it begins a phrase, not a complete sentence.
But handbooks are divided over whether to capitalize complete sentences.
The Chicago Manual of Style advises doing so only when the defining or expanding passage following the colon consists of two or more sentences.
Others disagree, and though I usually follow Chicago, I concur with them: It can be difficult in a passage to know when the definition or expansion ends, and the distinction between a single sentence and two or more seems trivial and inconsistent.
2. Setting up a quotation
He makes this moral argument: “Taking whatever we need from the world to support our comfortable lives is not worthy of us as moral beings.”