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: the volume of an object can be used to describe the size of that object. 
Fact 2: Mass and volume are the same thing.
the mass of an object can be used to describe the size of that object.

Fact 1: the ovaries are used in reproduction. 
Fact 2: Infertility is a disease of the reproductive system that prevents the conception of children.
The ovaries are used to have children.

Fact 1: using a gasoline engine requires burning gasoline. 
Fact 2: When they are burned, fossil fuels are consumed.

using a gasoline engine requires consuming gasoline.