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: detailed observation of celestial objects requires a telescope. 
Fact 2: Telescopes Telescopes Telescopes and microscopes both use two lenses to produce magnified images.

SOLUTION: Two lenses to produce magnified images allow for detailed observation of celestial objects.

PROBLEM: Fact 1: increasing heat causes thermal expansion. 
Fact 2: Motion helps to increase body heat.

SOLUTION: motion causes thermal expansion.

PROBLEM: Fact 1: producing thrust causes something to move. 
Fact 2: Another way to move a spacecraft is to thrust with gas jets.

SOLUTION:
spacecraft are propelled by thrust.