[CentOS] why i can not put my swap files in /dev?

Tue Jul 6 10:41:06 UTC 2010
Tang Jianwei <myhnet at gmail.com>

I tried to create some swap files in /dev directory for my desktop.
the dd and mkswap were ok. but when I try to swapon it, i get this:

# swapon /dev/myswap
swapon: /dev/myswap: Invalid argument

but when I mv the file to some other directory like /mnt or /, the 
swapon works.

could sb. tell me why?

Tang Jianwei