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.

Ex Input:
the channel catfish (scientific name:"ictalurus punctatus"), also called the channel cat, is a species of catfish.

Ex Output:
channel catfish


Ex Input:
aerospace engineering is a field of engineering that specializes in vehicles that move in fluids.

Ex Output:
aerospace engineering


Ex Input:
songkran is a festival held every year in thailand on april 13.

Ex Output:

songkran