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: a modest proposal is a famous work of satire by irish writer jonathan swift.
Example Output: a modest proposal

Example Input: a black powder rocket motor pushes a model rocket using black powder.
Example Output: black powder rocket motor

Example Input: the ankh was the egyptian hieroglyphic character that stood for the word ʿnḫ, meaning life.
Example Output:
ankh