Today's English lesson

From EnglishPlus:

I'm Ap-Positive
That A Colon
Should Go Here

Colons with Formal Appositives

Use a colon instead of a comma to introduce an appositive at the end of a sentence for emphasis.

Appositives may be words, phrases, or clauses. If it is an independent clause, that clause begins with a capital letter.

Correct: He was watching his favorite type of television show: a baseball game. (A comma is fine, but a colon here provides emphasis and/or formality.)

Correct: He learned a valuable lesson: Never argue with a woman. (The appositive here is an independent clause, so it is capitalized.)