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: Mutations may change the structure of a chromosome or just change a single nucleotide. 
Topic: mutations organism.
Most mutations are beneficial to the organism.

Fact: Nerve impulses are electrical in nature. 
Topic: Nerve impulses are electrical in nature.
Without myelin, nerves are unable to conduct their electrical impulses.

Fact: if the amount of available food and water decreases in an environment then animals may leave that environment to find food and water. 
Topic: food water nutrition.

Food and water provided to patients by mouth is the usual means of providing nutrition to patients.