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: børge rosenbaum (3 january 1909 – 23 december 2000), known professionally as victor borge , was a danish-american comedian, conductor, and pianist.
Example Output: victor borge

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: europe is a continent.
Example Output:
europe