Ruby: define_method vs. def

define_method is a (private) method of the object Class. You are calling it from an instance. There is no instance method called define_method, so it recurses to your method_missing, this time with :define_method (the name of the missing method), and :screech (the sole argument you passed to define_method).

Try this instead (to define the new method on all Monkey objects):

def method_missing(m)
    puts "No #{m}, so I'll make one..."
    self.class.send(:define_method, :screech) do
      puts "This is the new screech."
    end
end

Or this (to define it only on the object it is called upon, using the object’s “eigenclass”):

def method_missing(m)
    puts "No #{m}, so I'll make one..."
    class << self
      define_method(:screech) do
        puts "This is the new screech."
      end
    end
end