Adding functionality to objects by creating new small classes instead of adding and changing methods in old classes.

class Robot
  def initialize(name)
    @name = name 
  end
  
  def name
    puts @name
  end
end

class Aged < SimpleDelegator
  def initialize(source, age=0)
    super(source)
    @age = age
  end

  def age
    puts @age
  end
end

class Ranked < SimpleDelegator
  def initialize(source, rank=10)
    super(source)
    @rank = rank
  end

  def rank
    puts @rank
  end
end

class HumanFriendly < SimpleDelegator
  def age
    puts "My age is:"
    super
  end
  
  def rank
    puts "My rank is:"
    super
  end
  
  def name
    puts "My name is:"
    puts "Friend of Humanity"
  end
end

robot = HumanFriendly.new(Ranked.new(Aged.new(Robot.new("Destroyer of Humanity"))))
robot.name
robot.age
robot.rank

# output
# My name is:
# Friend of Humanity
# My age is:
# 0
# My rank is:
# 10