Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

Example Input: causality is a way to describe how different events relate to one another.
Example Output: causality

Example Input: language death is when a language stops being used as a first language (also called mother language or native language) and as a second language, which it then becomes an extinct language.
Example Output: language death

Example Input: sivaji ganesan was an actor of tamil cinema.
Example Output:
sivaji ganesan