A little example about open Classes in Ruby

try_my_dog.rb

class Dog
  def bark
    puts 'guau'
  end
end

def try_my_dog
  dog = Dog.new
  dog.bark
  begin
    dog.purr
  rescue NoMethodError
    puts "dog doesn't have a method called 'purr'"
  end
end

try_my_dog

class Dog
  def purr
    puts 'purr, purr, purr'
  end
end 

try_my_dog