Final answer:
A skilled writer uses a colon after an incomplete thought to introduce a formal list of items.
Step-by-step explanation:
A skilled writer uses a colon after an incomplete thought that introduces a formal list of items. The colon serves as a punctuation mark that indicates that a list is about to follow. For example:
The author went to the grocery store and bought all the ingredients for the recipe: flour, sugar, eggs, and butter.
In this sentence, the incomplete thought is 'The author went to the grocery store and bought all the ingredients for the recipe,' and the colon is used to introduce the list of items that were purchased.