What is an declarative sentence followed by incomplete question?

Here's an example of a declarative sentence followed by an incomplete question:

The sky is blue, why?

* Declarative sentence: "The sky is blue" - It makes a statement and ends with a period.

* Incomplete question: "Why?" - It's a question without a subject or verb.

Here are some other examples:

* The cake is delicious, what's the recipe?

* The dog is barking, who's at the door?

* The movie was scary, what happened next?