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 features of a production vehicle or production car are mass-produced identical models, offered for sale to the public.
Example Output: production vehicle

Example Input: euclid's lemma, also called euclid's division lemma or euclid's first theorem, is an important lemma.
Example Output: euclid's lemma

Example Input: a surfactant is a chemical that lowers the surface tension of a liquid.
Example Output:
surfactant