gistfile1.rb

class Animal
  def self.noise=(sound)
    @noise = sound
  end

  def self.noise
    @noise
  end

  def speak
    self.class.noise
  end
end

class Dog < Animal
  self.noise = 'Woof'
end

class Cat < Animal
  self.noise = 'Meeeeoowww'
end

> Cat.new.speak
=> "Meeeeoowww" 
> Dog.new.speak
=> "Woof"