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: sunlight and rain can cause a rainbow. 
Topic: rainbow sunlight.
Rainbows are caused by internal reflection and refraction of sunlight or moonlight inside raindrops.

Fact: Cells secrete acids and bases to maintain the proper pH for enzymes to work. 
Topic: enzyme catalyst.
An enzyme is a biological catalyst .

Fact: most canyons are formed by flowing rivers through erosion over long periods of time. 
Topic: rivers flows due to.

Rivers flow downhill due to gravity.