I've been working on this for quite some time and don't understand the error: C:\code> C:\code>type monsters.rb require 'Dragon' require 'ArrayOfCreaturesAttributes' require 'CreatureAttributes' monsters[0]=ArrayOfCreaturesAttributes.instance C:\code> C:\code>type ArrayOfCreaturesAttributes.rb require 'singleton' require 'Creature' require 'CreatureAttributes' class ArrayOfCreatureAttributes < Array include Singleton end C:\code> C:\code>type Creature.rb require 'CreatureAttributes' class Creature Creature.extend CreatureAttributes include CreatureAttributes end C:\code> C:\code> C:\code>monsters.rb C:/code/monsters.rb:6: undefined local variable or method `monsters' for main:Ob ject (NameError) C:\code>