The big idea
When you're writing about factual stuff, it's often a good idea to use a short quote from one of your sources to help you prove your point. That way, if your teacher questions a fact you use in your paper, you can use a quote to help make your case stronger. It's kind of like saying, "Listen: you don't have to take my word for it; here's what an expert on the topic says."
Remember, though: it's only okay to use somebody else's words in your paper if you do two things:
If you don't do these two things—especially the second one—you'll be guilty of plagiarism. This is absolutely not something that you want to do.
Before you begin
Make sure you've decided on a fact in your paper that you want to beef up with some expert opinion. It should be something that is not common knowledge.
How to do it
. . . .
An example:
A lot of people were negatively affected by the eruption of Mount Pinatubo. As Rosenberg writes, “Up to 800 people were killed and 100,000 became homeless following the eruption of Mount Pinatubo.” This shows how disruptive the eruption was to the people of the Philippines.