Liquids like saliva and milk do not dissolve teeth. In fact, it is widely recommended that you place a knocked-out tooth in saliva or, alternatively, milk until you can get to the dentist to have them try to replant the tooth. Teeth do dissolve in cola or soda pop. This is due to a number of factors. This makes cola acidic.
Acid dissolves teeth.
If the tooth is soaking in cola, the roots usually dissolve first because they lack enamel which covers the crown of the tooth to protect it from decay. As you know, even enamel will decay over time to cause a cavity. The carbonation may help to mix the cola. Try soaking one tooth in fizzy cola and another similar sized tooth in the same type and amount of flat cola. See which tooth dissolves the fastest to determine if carbonation is a major factor.