vehicles.rb

#!/usr/bin/env ruby
# vehicle.rb

class Vehicle
  MEDIUM = :ground
  def self.medium; self::MEDIUM; end
  # Using MEDIUM instead of self::MEDIUM makes Aircraft use the ground
end

class Car < Vehicle; end

module Flyable
  MEDIUM = :air
end

class Aircraft < Vehicle
  include Flyable
end

if (__FILE__ == $0)
  puts File.read(__FILE__)
  puts
  puts 'Produces:'
  puts
  [Vehicle, Car, Aircraft].each do |klass|
    puts "#{klass}.medium = #{klass.medium}"
  end
end