Does anyone know why this happens?

$ irb
irb(main):001:0> Dir.chdir '~'
=> 0
irb(main):002:0> Dir.chdir '~'
Errno::ENOENT: No such file or directory - ~
         from (irb):2:in `chdir'
         from (irb):2
$ ruby -v
ruby 1.8.1 (2003-12-25) [i686-linux]