Read the sentence from "black hole beginnings."
under the force of its own weight, the gigantic ball presses
inward.
what question should readers ask to determine the cause in
the sentence?
how much does the ball weigh?
why does the ball press inward?
what does the weight of the ball do?
how does the ball produce force?

taking into account that a theme is the central topic a text treats and can be developed through a story, play, poem etc. the following are themes that can be further developed:  

my best friend jeffrey has many unusual interests. (the story could be about each of the unususal interests)

once a man gives his soul over to evil, he can never go back. (this could be a saying or a proverb)

the three sheepdogs lay around the yard all day. (it could be developed detailing how each of these sheepdogs lay around)

a priest's collar has an unusual effect upon people. (it could be developed detailing the different effect upon people)

innocence is a powerful force in a world full of corruption. (it could be developed detailing how is a world full of corruption)

king lear is a play shakespeare wrote later. (it is just a sentence and doesnt give any idea or topic to be discussed in details.

evil often wears the disguise of innocence. (this could be a saying or a proverb)

because they were not just any seashells they were sally's seashells

pennfoster ? here you go

1. heat

2. acquitted, exonerated

3. rough

4. common

5. hate

6. harmful, toxic

7. public

8. apart

9. despise

10. careless

11. put

12. received

13. become

14. received

15. persuaded

16. make