module_function.rb

module Stuff
  def an_instance_method
    puts "Stuff's instance method!"
  end

  def self.an_eigenclass_method
    puts "Stuff's eigenclass method!"
  end

  module_function

  def a_module_function_method
    puts "Stuff's module function method!"
  end
end

Stuff.an_instance_method #=> error method missing
Stuff.an_eigenclass_method
Stuff.a_module_function_method

class Something
  include Stuff
end

thing = Something.new
thing.an_instance_method
thing.an_eigenclass_method     #=> error method missing
thing.a_module_function_method #=> error private method