AddInstanceVariable.rb

module Gorilla
  attr_accessor :gorilla

  #Init variable.
  def self.extended(obj)
    obj.gorilla = "First Gorilla"
  end
end

class Giraffe; end

#Add instance variable and accessor methods to this object. 
giraffe = Giraffe.new.extend(Gorilla)
giraffe.gorilla #=> "First Gorilla"