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

What's the name of Doctor Who's home planet ?
doctor who

The Battle of Poltava in 1709 was a victory by Peter the Great over which state?
battle of poltava

In 1907, which ill-fated liner of the Cunard Line made its maiden voyage?

cunard line