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: damming a river can cause a lake to form.
Example Output: beaver dam.

Example Input: Fact: a sandbar is formed by water moving sediment downstream.
Example Output: sandbar habitat.

Example Input: Fact: beads of water are formed by water vapor condensing.
Example Output:
Water vapor.