Why should the size of water pipe a person uses matter to law enforcement? It matters because if the bong water contains 4 fluid ounces or more of a mixture containing a controlled substance, the weight of the bong water can be used to increase the severity of the drug charge against you. Instead of being charged with a 5th degree controlled substance offense for residue found in a bowl, you could be looking at a prison commit when you add cocaine bong water to the weight of the residue. The bong water law went into effect in Minnesota on May 25, 2011 and applies to all crimes committed on or after that date. The new law does prohibits the use of the weight of the bong water if it is under 4 fluid ounces, which is equal to about a half of cup of water. My advice? A smaller bong is the best defense.