LM, Sek gave an excellent explanation - as always - but maybe something visual might help you. Let's play dominoes. The first domino is the first post in a thread. Let's hang that domino from the ceiling too. The next player (poster) comes along and hangs their domino (post) from the first one. For the most part, folks are usually hitting Quick Reply, Reply or Quote on the last post in the thread. So each domino is being hung directly under the lowest one and you have one long single column of dominoes. You can't remove a domino unless it does not have any dominoes hanging from it. Let's make it a little more interesting. Someone sees something in a post that is not the last post in the thread and they reply to it. Now we have someone hanging a domino off another that is not at the bottom. In the forum thread, that is now the last post that most folks are going to reply to. With the dominoes, we have to channel Salvidor Dali and have that domino stretch towards the floor until it is the lowest one. People normally hang their domino off the lowest one. If you get enough of these hung off dominoes that are not the lowest, you have something that starts to resemble a mobile in a baby's crib. As long as no one hangs a domino off the one you added, you are allowed to remove it if you wish. Your ability to delete a post is really determined by others, not you. In the 4-letter-word thread, folks are almost always replying to the last post so when your post is the newest (last) domino, as soon as someone posts after you, they are hanging a domino from yours and now you can't remove your domino anymore.