How does my ipod know?

I had this thought while running two Saturdays ago - how does my ipod know how to motivate me? When I start my run, I set my ipod to shuffle and just listen to whatever comes out (unless it's some love song Erin put on there). Here's what came out in one great stretch (I only ran 3 miles): Green Day, Nirvana, Foo Fighters and Weezer. But just as I finished stretching, "Paper Wings" from the Hope Floats Soundtrack came on, one of Erin's songs. Maybe my ipod knows I love 90's alternative because that's all I put on there.