class_extension.rb

# ======================
# Spell: Class Extension
# ======================

# Define class methods by mixing a module into a class’s eigenclass
# (a special case of Object Extension - http://gist.github.com/534667).

class C; end

module M
  def my_method
    'a class method'
  end
end

class << C
  include M
end

C.my_method # => "a class method"

# For more information: http://www.pragprog.com/titles/ppmetr/metaprogramming-ruby