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: a planet rotating causes cycles of day and night on that planet. 
Fact 2: Rotation determines day and night, and the length of the day.

Answer: a planet's rotation is what causes night.


Question: Fact 1: Light enters the eye through an opening called the pupil. 
Fact 2: Light is essential to vision.

Answer: vision requires the pupil.


Question: Fact 1: nuclear reactions in stars causes stars to produce light. 
Fact 2: Earth is one of the planets orbiting a star called the Sun.

Answer:
Nuclear reactions provide the Earth with light.