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: a cavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone. 
Topic: carbonic acid.
Any time water and carbon dioxide mix, carbonic acid is the result.

Fact: Sperm pass out of the body through the urethra. 
Topic: urethra tube penis.
Urine leaves the bladder in a single tube, the urethra, that runs to the penis or to the vagina.

Fact: Mammals generate heat mainly by keeping their metabolic rate high. 
Topic: high metabolic rate.

Birds have a high metabolic rate and a high energy requirement.