The purpose is to be able to get the coil very cold without freezing. IMPORTANT don't use water inside the coil bong! If you use water then put in the freezer, it will blow the coin(button) off, when the water starts to freeze from or or if you use a very strong adhesive on the button the glass itself will crack from the expansion. You can take off the upper part to clean you bong easy. Extremely useful! http://www.smokewire.com/glycerin-coil-bong.html
