String class does not call to_s when it is representing its own objects. So Kernel#puts which generally calls an objects to_s method does not do so when the object inherits from String.

unpredictable_string.rb

class UnpredictableString
  def initialize string
    @self_badname = string
  end

  def scramble
    @self_badname.split(//).shuffle.join
  end

  def to_s
    scramble
  end
end

my_unpredictable = UnpredictableString.new('This is unpredictable')
5.times do
  puts my_unpredictable #if UnpredictableString inherits from String the puts returns nil.
end