Monkey Patching

gistfile1.rb

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

module Monkey
  def self.included(base)
    base.send(:include, InstanceMethods)
    base.class_eval do
      alias_method :bar, :meow
    end
  end

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

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