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 calculus of variations is a field of mathematical analysis.
Example Output: calculus of variations

Example Input: spintronics is a kind of electronics that uses the tiny magnetic fields that come from electrons as well as their electric charge.
Example Output: spintronics

Example Input: parasailing is a sport.
Example Output:
parasailing