class_inheritance.rb

module Animal
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def say
      "animal"
    end
  end
end

class Slug
  include Animal
end

class Cat
  def self.say
    "meow"
  end
end

class Dog
  class << self
    def say
      "woof"
    end
  end
end

Slug.say
Cat.say
Dog.say

Cat.send(:include, Animal)
Cat.say
Dog.send(:include, Animal)
Dog.say