instruction:
Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.
question:
The film The Curious Case of Benjamin Button, starring Brad Pitt, is based on a short story by whom?
answer:
the curious case of benjamin button

question:
Which Beatles song was also recorded by Peter Sellers?
answer:
the beatles

question:
Dr. Who's companion as played by Billie Piper?
answer:
billie piper