nyan.rb

module Nyan
  def nyan
    "nyan"
  end

  extend self
end

class Neko
  include Nyan

  def mew
    nyan
  end
end

p Nyan.nyan # => nyan
p Neko.new.mew # => nyan
p Neko.new.nyan # => nyan

piyo.rb

module Piyo
  def piyo
    "piyo"
  end

  module_function :piyo
end

class Hiyoko
  include Piyo

  def chun
    piyo
  end
end


p Piyo.piyo # => piyo
p Hiyoko.new.chun # => piyo
p Hiyoko.new.piyo # => private method `piyo' called NoMethodError

wan.rb

module Wan
  class << self
    def wan
      "wan"
    end
  end
end

class Inu
  include Wan

  def bow
    wan
  end
end

p Wan.wan # wan
p Inu.new.bow # => NoMethodError
p Inu.new.wan # => NoMethodError