Just put an 'if' condition with what to do if the condition was true in the 'act' method of the class of the rain objects. If you are unsure of what methods to use, first start in the Actor class (since the raindrops are Actor objects). The key words or phrases that you used above are 'world', 'disappear', 'and 'bottom of world'. Since 'world' is in there, you should probably look at the methods available in the World class as well as looking for a method in the Actor class that returns the World object it is in.