Module magic with extend self, and extending others

irb

irb(main):001:0> Bar.bar
=> :barred
irb(main):002:0> Bar.new.bar
=> :barred
irb(main):003:0> Foo.bar
=> :barred

silly.rb

module Foo

  extend self

  def self.included(base)
    base.extend(self)
  end

  def bar
    :barred
  end

end

class Bar
  include Foo
end