How can you put songs from your iTunes into your friends iPod with erasing their music?

Once you've connected your iPod to the computer click on your iPods name on the left hand side. A picture of an iPod should show up and a the selected tab at the top should say 'Summary'. At the bottom of the page there will be a section that says 'Options'. On one of the boxes it will say 'Manually Manage Music', click that box so that a check mark shows up. Press the Apple button and let it sync. When you go to your friend's library all you have to do is drag the songs you want to your iPod. Next to your iPods name two arrows will appear that go in circles and you're done.