A complete simple sentence tells a complete thought. It has a subject, or naming part, and a predicate, or action part.

Sometimes writers run two sentences together without using punctuation. This is called a run-on sentence.

There are two complete thoughts in the example run-on sentence in the graphic, which means there are two sentences. The sentences were not separated by punctuation.

To fix the sentence, write:

I went to a museum. My sister went to the movies.

We had to add a period after museum and a capital letter to my to start the new sentence.