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.

Q: Fact 1: telephones are used for communication by two people by transmitting sound. 
Fact 2: All sounds are waves produced by the vibrations of material objects.

A: Telephones are used for communication by two people by transmitting waves produced by the vibrations of material objects.
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Q: Fact 1: energy is used for heating a building by a heating system. 
Fact 2: Energy used for heating is the single most commonly used form of energy worldwide.

A: Heating buildings is a very common production of energy worldwide.
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Q: Fact 1: Bird eggs have hard, calcium carbonate shells. 
Fact 2: An egg shell is mostly calcium carbonate.

A:
bird eggs are hard shells.
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