What has happened to my baby tooth?

Unhealthy eating habits can contribute to tooth decay, but can also appear when the child's teeth and gums are exposed to any liquid or food other than water for long periods or frequently throughout the day. Natural or added sugars in the liquid or food are changed to acid by bacteria in the mouth, this acid then dissolves the outer part of the teeth, causing them to decay. I'd definitely get this checked out by a dentist!