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.

Fact: sound can be used for communication by animals.
Sound is frequency.

Fact: Any antigen that causes an allergy is called an allergen.
allergen.

Fact: a black hole is formed by a high-mass star collapses.

black hole.