In this task, you need to write a topic word from the given fact. The topic word must have at least one word overlap with the given fact. The topic word often involves adding a new word from a related concept. In your topic word, use at least one word from the given fact. Topic words with two or more words work best.

Example Input: Fact: propulsion is used for flying by a spacecraft.
Example Output: propulsion spacecraft.

Example Input: Fact: hydropower requires damming a river.
Example Output: damming river.

Example Input: Fact: Antigens are found on cancer cells and the cells of transplanted organs.
Example Output:
antigen cancer.