concern.rb

module Refillable
  extend ActiveSupport::Concern
  
  include do
    attr_reader :store
  end
  
  def refill!
    @store = 100
  end
  
  def needs_refill?
    store.nil? || store < 5
  end
  
  module ClassMethods
    def overstretched
      "this example"
    end
    
    private
    
    def something
    end
  end
end