ipod.rb

class Battery
  attr_reader :level
  
  def initialize
    @level = 0
  end
  
  def charge(uplevels)
    @level += uplevels
    @level = 100 if @level > 100
  end
  
  def use(downlevels)
    @level -= downlevels
    @level = 0 if @level < 0
  end
end

class IPod
  attr_reader :battery
  
  def initialize
    @battery = Battery.new
  end
end

iPod = IPod.new
iPod.battery.level # => 0
iPod.battery.charge(80)
iPod.battery.level # => 80