The glycerin bong is a type of water pipe that consists of glycerin-filled chambers or freezable coils to cool the smoke before it is inhaled. Phoenix star glycerin bongs are filled with medical grade glycerin which is safe and does not expand after freezing. 
The glycerin chamber or glycerin coil is typically detachable and can be pre-frozen in the refrigerator for 1-2 hours or even more to enhance its cooling effect, allowing the glass bong to provide a smoother and cooler smoking experience.