Water pipes do a lot to cool down smoke just by making the hot, ash filled air be filtered through water, although to many people the smoke created is still too harsh.
Many water pipes, such as the UPC "Simply Guy," utilize an ice pinch, which is a tried and true method of cooling the smoke even more after it has passed through the water. Ice is a wonderful way to cool hot smoke making it condense to create a more powerful hit while also making it easier to inhale, especially for those with sensitive throats or lungs.
Ice is very nice when you are looking for a cool smoke, but it has a few drawbacks as well. When using ice in an ice pinch, the inevitable melting will cause the water level to rise and that could cause a reduction in functionality of the piece and increases the chance of getting water in your mouth.
There is a solution: Glycerol, also known as glycerin, is a simple compound made of sugar alcohol with a wide variety of useful attributes. Glycerin is useful to the functional glass industry because it has an extremely high freezing point, meaning that it can be cooled down to extreme temperatures without freezing and expanding.
Water pipes that utilize glycerin have a hollow cavity somewhere on the piece that is filled with glycerin and then sealed either with a rubber stopper or a small amount of molten glass is melted over the hole. Water could not be used for this purpose because it would expand in the freezer and crack the glass that houses it.
Glycerin is a great option for cooling down smoke, and the geniuses at work are consistently coming up with new and exciting ways to incorporate it into smoking accessories. Remember, Ice is nice, but glycerin is better!