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.

Fact 1: skin is used for breathing air by frogs. 
Fact 2: Air contains oxygen.
skin is used for breathing oxygen by frogs.

Fact 1: Muscle contraction occurs when muscle fibers get shorter. 
Fact 2: Isometric exercise is a sustained contraction of large muscles, such as weight lifting.
Weight lifting requires  shortening of of the fibers of large muscles.

Fact 1: Sweat wets the skin, and when it evaporates, it cools the body. 
Fact 2: Fig 4-23 sweat contains water, salt, lactic acid, vit.

evaporation of water cools the body.