ProTip™ on Constant Scoping and Resolution in Ruby: One of these is not like the other.

scoping_in_ruby_19.rb

module Root
  FOO = 'Ayyyy!'
end

module Root::NotNested
  def self.speak
    puts FOO
  end
end

module Root
  module Nested
    def self.speak
      puts FOO
    end
  end
end

Root::FOO              #=> 'Ayyyy!'
Root::NotNested.speak  #=> NameError: uninitialized constant Root::NotNested::FOO
Root::Nested.speak     #=> 'Ayyyy!'

:) For my personal reference:
http://stackoverflow.com/questions/6927566/using-instead-of-module-for-ruby-namespacing
http://stackoverflow.com/questions/15119724/ruby-lexical-scope-vs-inheritance