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Gallstones form when liquid stored in the gallbladder hardens into pieces of stone-like material.
The liquid, called bile, is used to help the body digest fats. Bile is made in the liver, then stored in the gallbladder until the body needs to digest fat. At that time, the gallbladder contracts and pushes the bile into a tube called a duct that carries it to the small intestine, where it helps with digestion. Bile contains water, cholesterol, fats, bile salts, and bilirubin. Bile salts break up fat, and bilirubin gives bile and stool a brownish color.