animal.rb

class Animal
  def set_from_animal
    @@x = :animal
  end
  def get_from_animal
    @@x
  end
end

class Dog < Animal
  def set_from_dog
    @@x = :dog
  end
  def get_from_dog
    @@x
  end
end
  

distinct.rb

require 'animals'

Dog.new.set_from_dog
Animal.new.set_from_animal

# Animal and Dog have distinct @@x variables
puts Animal.new.get_from_animal
puts Dog.new.get_from_dog

Example Output

$ ruby shared.rb 
dog
dog
$ 
$ ruby distinct.rb 
animal
dog
$ 

shared.rb

require 'animals'

Animal.new.set_from_animal
Dog.new.set_from_dog

# Animal and Dog share an @@x variable
puts Animal.new.get_from_animal
puts Dog.new.get_from_dog