Given a factoid/trivia type question, generate the topic of the question. The topic is the entity the question talks about.

Which SF author wrote The Day it Rained Forever?
the day it rained forever

Which British composer wrote the music used as the theme for Desert Island Discs  He also wrote the Dambusters march and the Knightsbridge march.?
the dam busters

Zeus' visit as a swan to whom resulted in the birth of twins Castor and Pollux?

castor and pollux