1.2 Run-on Sentences: Introduction

Take a look at this example of a sentence that adds on without running on. Notice the pattern it follows. In this sentence, what is the core sentence (independent clause)?

George sleeps on the couch, stretching his little paws out every once in while.

Take a look at this example of a sentence that adds on without running on. Notice the pattern it follows. In this sentence, what is the dependent(s) (additions attached and grouped with commas)?

George sleeps on the couch, stretching his little paws out every once in while.

Take a look at this example of a sentence that adds on without running on. Notice the pattern it follows. Why is there a comma after "George sleeps on the couch"?