a very simple question about in ear headphones

I bought mine 2nd hand (as I was NOT willing to drop 80 bucks for them) and they did not come with a manual. Didn't think that would be a problem - until I got them ...I can not, for the life of me, figure out how to tell it to go to the next song. I see the up & down volume and the middle button pauses it.... how do I advance it????? Supposably, I can - but how?!?!?!?!