A discussion list in unlike a normal list in that it consists of two content types - a Discussion content type that maps to the discussion topic and a Message that maps to all replies to the topic. If you examine the Discussion content type you’ll see that its inherited from the Folder content type. The reply contains a field called ‘ParentFolderId’ and as the name suggests it holds the ID of the discussion topic. So naturally if you were to do a query on the list for all items with the same ParentFolderId...