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.

Fact: Gene flow occurs when individuals move into or out of a population. 
Topic: migration move called.
Internal migration describes moves within a country.

Fact: Food consists of organic molecules that store energy in their chemical bonds. 
Topic: chemical bonds.
Chemical bonds are electrical in nature.

Fact: treating water is used to remove harmful substances before drinking. 
Topic: boiling water treat.

Boiling is the most reliable way to treat contaminated water.