Another thing that you can do, which is a little different, is to highlight the passage of someone else's text that you want to quote and copy it with your mouse (or by presing the control button and the "c" button at the same time on your keyboard). Place (paste) the text into your message box and wrap quote tags around the quoted text.
This can be done by highlighting the text that you want to have as a quote and then selecting the icon from the messages tools that looks like a talking bubble (little square with words on it). Once you press this, your highlighted text will become a quote. However, what makes this different from the really handy method that Purrfectlydevine explained is that when you decide to quote your text with the method that I just described it does not list the name of the original poster (person who's text you are quoting). So it can work well if you are replying directly after someone's comment, but if you are quoting something that was said 7 o 8 posts ago you may want to include the name of the person whom you are quoting.
Both methods work well, the one that Purrfectlydevine explained is certainly a bit faster and handier though,and perhaps a tad quicker too
(Below is an example of the method that I described.)
How do you get sections of a person's post copied into YOUR post so that everyone knows what you are replying to? Those blue-gray boxes that most of you have in your replies to a particular person ... how do I get those?