format error
 

Hi. I saved my music files as .mp3 and put them into the mp3 player (i have a 4gb iPod nano knockoff) and when I click on them the player says "format error!" and doesn't play them. has anyone else encountered this problem, and how can I fix it? is there something else i should be saving the files as... because I think .mp3 is supposed to be the correct format, right? please help!