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: boiling means change from a liquid into a gas by adding heat energy.
Example Output: boiling change.

Example Input: Fact: Cells vary in size as well as shape, but all cells are very small.
Example Output: Cells are functional unit.

Example Input: Fact: if a neutral atom loses an electron then an atom with a negative charge will be formed.
Example Output:
atom electron.