After you click on "reply", click on the greyish black reectangle to the top right of the message typing box and click on where it says "quote". The other person's message will show up so that you can more easily reference what you're repsonding to.
@sprocket Just a further note to that:
If you have enabled "Quick Reply" in your personal settings, using that button when replying will not give you the option to quote the previous message to which you're replying. Only the regular "Reply" button allows that.
Nailed it. I tried it out and quickly realized I couldn't easily do a quote. Right on.