I'm wondering what is in it that rots out the teeth?

In what ?: What is in what that destroys your teeth? Acid that bacteria produce softens the enamel and then the dentin.

Cavities: Cavities, holes in teeth, occur when bacteria convert sugars/starches into acids. The acids etch calcium out of the hard tooth enamel, allowing bacteria to infect the softer dentin under the enamel, the "nerve" of the tooth under the dentin, and then spread through the circulatory system to gums, bone, brain, heart, etc. Please see your dentist regularly and brush/floss as directed.