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.

Input: Consider Input: Fact: when electricity flows to a light bulb , the light bulb will turn on.

Output: electricity electrons.

Input: Consider Input: Fact: Cells vary in size as well as shape, but all cells are very small.

Output: Cells are functional unit.

Input: Consider Input: Fact: a meter stick is used to measure distance.
Output: "meter stick".