Milk tooth. Coloured scanning electron micrograph (SEM) of a milk tooth. The root of this tooth has been dissolved, a process known as resorption, leaving just the crown, the visible part of the tooth. Milk teeth, also known as deciduous teeth, are the first set of teeth in mammals. Resorption occurs during exfoliation, the replacement of the milk teeth by the permanent teeth. The permanent tooth presses down on the root of the milk tooth, causing it to dissolve. The dissolved material is then absorbed by the permanent tooth.