#from http://pragmaticprogrammer.com/titles/ruby/code/smp382.html
# comments added

  a = "hello"
  b = a.dup
  
  class <<a                                # associates a singleton class with object a
                                                   # it becomes the klass object for a
                                                   # it's superclass is String
    def to_s
      "The value is '#{self}'"
    end
    def two_times
      self + self
    end
  end

p a.to_s
p a.two_times
p b.to_s                                    # the klass object of b is still String