MonkeyPatching

gistfile1.rb

class Foo
  def self.bar
    puts "Hello, world!"
  end
end

module Monkey
  def self.included(base)
    base.extend(ClassMethods)
    base.class_eval do
      class << self
        alias_method :bar, :meow
      end
    end
  end

  module ClassMethods
    def meow
      puts "Meow!!"
    end
  end
end

Foo.bar # => "Hello, world!"
Foo.send(:include, Monkey)
Foo.bar # => "Meow!!"