class Animal
  def method_missing(method, *args, &block)
    say, speak = method.to_s.split("_")
    if say == "say"
      speak
    else
      super
    end
  end
end

=> a = Animal.new
=> a.say_moo
=> "moo"
=> a.shout_moo
=> NoMethodError: undefined method `shout_moo'