Tooth erosion occurs when the enamel on your teeth is worn away by acid. Usually the calcium contained in saliva will help remineralize (or strengthen) your teeth after you consume small amounts of acid, but the presence of a lot of acid in your mouth does not allow for remineralization. Acid can come from many sources, including:
Carbonated drinks. All "fizzy" drinks, including soda (even diet varieties), contain a lot of acid and can dissolve enamel on your teeth very quickly. More damage is done when you drink large amounts and or you hold the drink in your mouth for a long time.
Pure fruit juice. Juice has similar effects on your teeth because it contains a lot of acid.
Bulimia and acid reflux. Bulimia, a disease in which a person vomits to avoid gaining weight, and acid reflux also can cause tooth damage due to stomach acids. Medical and dental help should be sought immediately if you or anyone you know suffers from a condition such as this.