Ruby Trivia

wag_the_dog.rb

class Dog
  def wag(tail=my_tail)
    tail
  end
  def my_tail
    :dogs_tail
  end
end

class Cat
  def wag_dog
    Dog.new.wag
  end
  def my_tail
    :cats_tail
  end
end

puts Cat.new.wag_dog          # What is printed here?
                              # Make a prediction before you try it.

Putted even more simply it's all about this:

Foo = Class.new do
  def bar(x=self)
    x
  end
end

Foo.new.bar