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.

Input: Consider Input: the bighorn sheep ("ovis canadensis") is a species of sheep.

Output: bighorn sheep


Input: Consider Input: causality is a way to describe how different events relate to one another.

Output: causality


Input: Consider Input: an elliptical galaxy is a galaxy having an ellipsoidal shape, and a smooth, nearly featureless brightness profile.


Output: elliptical galaxy