How does sugar damage teeth?: Dr Ian Kelly, Dentist
In simple terms, sugar is converted to acid by the bacteria in the mouth, and the acid will demineralise, or remove the calcium from the enamel, which weakens it.
Eventually the enamel can't sustain itself and collapses, which exposes the soft underbelly of the tooth - i.e. the dentine. Similar bacteria will then move into the tooth and will eat away at the dentine, which is basically the creation of a cavity or decay.