Cleaning iPod's HD

Just a quick question here. How would one go about totally erasing an iPod's hard drive? Like when people sell them on ebay, how do they erase the date to when it was like when they first bought it? I read this somewhere but can seem to remember.

Thanks for any info.

 

Thanks for the prompt reply!

Do you plug the iPod into the computer, then run the updater? Then from there it'll detect the iPod and ask me what I want to do?

Thanks again

 

Thanks!