why some packages are kept back?

["Yuwen Dai" <yuwend@gmail.com>]

Dear all,

I'm using Debian Sarge. When I ran apt-gete upgrade, this message shows:

  ~# apt-get upgrade
  Reading Package Lists... Done
  Building Dependency Tree... Done
  The following packages have been kept back:
    ffmpeg libmjpegtools0 mjpegtools mplayer-586 thunderbird
  0 upgraded, 0 newly installed, 0 to remove and 5 not upgraded.


Why?  What should I do?

-- 
Yuwen