extend.rb

module Helpers
  def help
    puts 'helpful!'
  end
end

class Foo

  def get_help
    self.extend Helpers
    help # will work because this instance now has Helpers.help
  end

  def fail
    help # will raise exception even after #get_help runs
  end
end

Foo.new.get_help # prints 'helpful!'
Foo.new.fail     # raises exception even after that last instance got extended with Helpers