The next Bloons Tower Defense game will have an Idle Mode. In this mode, all that will be seen on the screen will be a single dart monkey facing a very short track on which one red bloon will come every five seconds. With this mode on, you can leave the game open in a tab or window while doing something else on your computer (or while doing something else entirely), and still gain experience that will help you to rank up. There will also be some Achievements that can only be unlocked in Idle Mode.