Dog Rocks filter out impurities from water such as Tin, ammonia and nitrates. These impurities are usually passed out through urine, and when grass comes in contact with these elements it results in urine burn on grass, resulting in a dead, yellow patch on your lawn. Dog Rocks give your four legged friend a cleaner source of water, while eliminating urine burn on grass this will result in the stop of those nasty burnt yellow patches on your lawn. Without the impurities in the water, Dog Rocks actually help your dog fertilize your lawn! This is great for you, your dog & your lawn!