gistfile1.rb

module TheExtended
  
  def self.extended(extensor)
    raise "WHERE IS THE THING!?!?" unless extensor.constants.include?('THE_THING')
    extensor.instance_eval "
      def a_thing
        #{extensor.name}::THE_THING
      end
    "
  end

  def using_the_thing
    "I use #{a_thing}!"
  end
end


module Extender
  THE_THING = "the thing"
  extend TheExtended
end

puts Extender.using_the_thing

gistfile2.rb

module TheExtended
  
  def self.extended(extensor)
    raise "WHERE IS THE THING!?!?" unless extensor.constants.include?('THE_THING')
    extensor.instance_eval "
      def a_thing
        '#{extensor::THE_THING}'
      end
    "
  end

  def using_the_thing
    "I use #{a_thing}!"
  end
end


module Extender
  THE_THING = "the thing"
  extend TheExtended
end

puts Extender.using_the_thing

Or this thing

module TheExtended
  def using_the_thing
    "I use #{a_thing}!"
  end
  
  def a_thing
    raise "O NOSE!"
  end
end


class Extender
  THE_THING = "the thing"
  extend TheExtended
  
  def self.a_thing
    THE_THING
  end
end

puts Extender.using_the_thing