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: thirst is used to make an animal realize that it needs to replenish its bodily water by the body. 
Fact 2: Every plant and animal needs water to survive.

SOLUTION: thirst helps animals to survive.

PROBLEM: Fact 1: Echinoderms use pheromones to communicate with each other. 
Fact 2: Modern echinoderms include starfish and sea urchins.

SOLUTION: Starfish use pheromones to communicate with each other.

PROBLEM: Fact 1: erosion causes a river to become deeper and wider. 
Fact 2: Canyon formed as a consequence of river erosion.

SOLUTION:
Canyons are formed as rivers become deeper and wider.