Finally, I figured it out and it does not involve arcane HTML. The TT4 software interprets square brackets [ and ] as tags for a link, so if you post something in brackets, the brackets disappear and your words show up on blue, as a link to the Thorntree main page. So you cannot post that "Thronetree [sic] is my favorite web site."
Ah, but you can. Put a backslash () in front of each bracket. It works for <strong>asterisks</strong> as well.
