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 warm front causes cloudy and rainy weather. 
Fact 2: If warm air is advancing on cold air, the front is designated a warm front.

SOLUTION: Warm air advancing on cold air can lead to cloudy and rainy weather.

PROBLEM: Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers cause erosion by transporting weathered rock and soil downstream.

SOLUTION: Valleys are formed by erosion.

PROBLEM: Fact 1: beads of water are formed by water vapor condensing. 
Fact 2: When water vapor condenses, energy in the form of heat is given to the remaining air molecules.

SOLUTION:
When water vapor condenses heat energy goes to the remaining air molecules.