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:
a rearguard is a part of a military force that protects the main force from attack from the rear.

Ex Output:
rearguard

Ex Input:
the melbourne storm are an australian professional rugby league team.

Ex Output:
melbourne storm

Ex Input:
corrosion is the breakdown of materials due to chemical reactions.

Ex Output:
corrosion