Why does my iPhone start playing music when my headphones are half way plugged in?

Whenever I plug my headphones in to my first generation iPhone, when they are about halfway in, music starts playing! It always starts on a playlist that is created on my phone. Is this normal?|||Does it play where you left off last time. If yes, what's happening is that the iphone headphones have a mic which is also a remote that controls the iphone playback. In this case, when you plug in your headphones halfway, it sends a signal to the iphone that the button for playback has been pressed. Either there's dirt in the headphone jack(try blowing some air), or your headphones have a short. Try different set of headphones.