Kindle is incapable of opening .mobi files, its own file format, directly. If you download a .mobi file, and attempt to open it thinking "i have kindle, and this is a kindle specific file format" you will be greeted with"You don't have any apps that can read this type of file". in order to open its own file format, you have to either email the file to your kindle email (which doesn't have an inbox) or you have to use a computer to drag and drop the file into Kindles documents folder.