The Colon Introducing an Example or Elaboration

A colon works like a drumroll. 'He had one rule: never look down' sets up an expectation with the first half, then the colon says watch this, and the second half pays it off. That's the whole job of this kind of colon: it joins a complete statement to whatever explains, lists, or illustrates it. The first half has to be a full sentence on its own, because it's the part doing the promising. If you can't stop there and have a complete thought, the colon is wrong.

The second half is a different story. It can be a full sentence, a phrase, or even a single word, because its only job is to deliver, not to stand alone. 'Only one thing stopped her: fear of heights' works fine with just a noun phrase after the colon. That's also what separates a colon from a semicolon here: a semicolon needs two independent clauses on both sides, but a colon only needs one on the left.

This topic sits next to semicolons and dashes because all three can join related ideas, and the test likes to make you choose between them. The test for a colon is simple: read only the words before it. If they form a complete sentence, and what follows explains or exemplifies them, the colon is right. If the left side is incomplete, cross the colon off your list immediately.