The purpose of a bong is to remove impurities from your herbal mixture (if you are good and use it according to directions) or from your marijuana (if you are a deranged substance abuser). So bong water naturally ends up with all of these impurities either in solution or precipitating. I cannot overemphasize the importance of regularly changing your bong water. Ideally, you would do so every time you smoke; that's why I smoke my herbal mixture in the bathroom.