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: the boiling point of a substance is the temperature at which the substance boils, or enters a state of rapid evaporation.
Example Output: boiling point

Example Input: aillant-sur-tholon is a commune.
Example Output: aillant-sur-tholon

Example Input: gliese 667 cc is an extrasolar planet 23.62 light years away.
Example Output:
gliese 667 cc