问TypeError:超类与Ruby中的class Word不匹配

EN

irb(main):016:0> class Word
irb(main):017:1>  def palindrome?(string)
irb(main):018:2>   string == string.reverse
irb(main):019:2>  end
irb(main):020:1> end
=> nil
irb(main):021:0> w = Word.new
=> #<Word:0x4a8d970>
irb(main):022:0> w.palindrome?("foobar")
=> false
irb(main):023:0> w.palindrome?("level")
=> true
irb(main):024:0> class Word < String
irb(main):025:1>  def palindrome?
irb(main):026:2>    self == self.reverse
irb(main):027:2>  end
irb(main):028:1> end
TypeError: superclass mismatch for class Word
 from (irb):24
 from C:/Ruby193/lib/ruby/gems/1.9.1/gems/railties-3.2.1/lib/rails/commands/console.rb:47:in `start'
 from C:/Ruby193/lib/ruby/gems/1.9.1/gems/railties-3.2.1/lib/rails/commands/console.rb:8:in `start'
 from C:/Ruby193/lib/ruby/gems/1.9.1/gems/railties-3.2.1/lib/rails/commands.rb:41:in `<top (required)>'
 from script/rails:6:in `require'
 from script/rails:6:in `<main>'