File/directory permission question

[Peter Blajev <peter castandcrew com>]

Hi,

I created the following directory structure and file aaaaa:

[peter sdpeter asdf]$ ls -al
total 8
drwxrwxrwx    2 peter    peter        4096 Dec  2 20:02 .
drwxrwxrwx    8 peter    peter        4096 Dec  2 20:00 ..
-rw-rw-r--    1 peter    peter           0 Dec  2 20:02 aaaaa

Then I decided to chown the file and got "Operation not permitted"

[peter sdpeter asdf]$ chown gregory aaaaa 
chown: changing ownership of `aaaaa': Operation not permitted


What is going on here?
It's my file. Shouldn't I be able to do whatever I want with it?

Another example:

   4 drwxrwxrwx    2 peter    users        4096 Dec  2 20:02 .
   4 drwxrwxrwx    8 peter    users        4096 Dec  2 20:00 ..
   0 -rw-r--r--    1 gregory  users           0 Dec  2 20:02 aaaaa

Why peter can not chmod file aaaaa?

It looks like I'm missing some basic linux knowledge here.

Please help.

Thanks
Peter