Cavities in a child's milk teeth. Coloured dental X-ray showing multiple filled cavities in the milk (primary) teeth of a 4-year-old child. Cavities, or caries, are caused by acid produced by bacteria as they ferment food debris accumulated on the tooth surface, resulting in demineralisation and destruction of the hard tooth tissues. The child's permanent teeth can be seen embedded in the jaw waiting to push through once the milk teeth have fallen out.