Write a fact related to the given fact, based on the given topic word. Note that, your fact should have at least one word in common with the given fact. All facts in this task refer to scientific facts. Your related fact must form a chain with the given fact. Chains form when two facts connect together to produce the third fact. An example of a chain is: "pesticides cause pollution" (given fact) + "pollution can harm animals" (related fact) → "pesticides can harm animals" (connected chain). Avoid creating simple paraphrases of the given fact. While your generated fact should be related to the input fact, they must describe slightly different scientific phenomena. It's okay if your related fact includes some irrelevant information, provided that it has some overlap with the given fact and it contains some words corresponding to the provided topic.

Q: Fact: when sound reaches the ear , that sound can be heard. 
Topic: hear sounds.

A: For sound, the sense of hearing is used.
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Q: Fact: microorganisms can cause infections. 
Topic: infection.

A: Bacteria are microorganisms that exist everywhere.
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Q: Fact: Population density is the average number of individuals per unit of area or volume. 
Topic: population density overpopulated.

A:
Another measure of growth is population density.
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