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: Cells vary in size as well as shape, but all cells are very small. 
Fact 2: Basic unit of the plant body is the cell.
Basic units of the plant body vary in size as well as shape, but all are very small.

Fact 1: using a gasoline engine requires burning gasoline. 
Fact 2: Gasoline and flammable liquid related burns, however, are a preventable problem.
using an engine requires burning flammable liquid.

Fact 1: breathing is when a gill converts from oxygen in water into oxygen in blood. 
Fact 2: All fish have gills .

Fish breathe by converting oxygen in water into oxygen in blood.