instruction:
Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
question:
Fact 1: Many corals secrete an exoskeleton that builds up to become a coral reef. 
Fact 2: Coral is calcium carbonate with a trade of carotene.
answer:
calcium carbonate becomes a  reef.


question:
Fact 1: plasma is formed by electrons separating from atoms in stars. 
Fact 2: Most of the matter in stars is in a plasma state.
answer:
electrons separate in stars.


question:
Fact 1: smell is used for finding food by some animals. 
Fact 2: Food includes small rodents and birds.
answer:

If an animal likes to eat rodents, they may use smell to find them.