It's the sugar that rots your teeth. Plaque contains sticky substances that absorb the sugar, and bacteria turn the sugar into acids that attack the tooth enamel. The carbonic and citrus acids in fizzy drinks, plus phosphoric acid (found in Coke) are water-soluble, and since saliva will wash these away quite rapidly, they aren't much of a problem. It's the acids that quench your thirst. (e.g. Sucking a toffee won't). For a really effective thirst-quenching drink, try a teaspoonful of cider vinegar in a glass of water. You'll find that a couple of mouthfuls is all you'll need. (N.B. It must be cider vinegar)