TASK DEFINITION: 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.
PROBLEM: Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers erode the rocks they flow over.

SOLUTION: Water erodes rocks as they flow over, creating a valley.

PROBLEM: 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.

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

PROBLEM: Fact 1: a calculator is used for adding. 
Fact 2: Calculator performs basic arithmetic, such as addition and subtraction.

SOLUTION:
arithmetic includes uses for subtraction.