There still are nerve endings in the tooth hidden beneath the dentin. The dentin is a hard, dense bony tissue forming the bulk of the tooth beneath the enamel. When the nerve endings are exposed to air or to hot or cold food or drinks, it can cause extreme discomfort. The nerve endings can be exposed when the tooth chips, cracks or breaks. These can happen in several ways like biting down on something hard, being hit in the face, falling, having cavities that weaken the tooth or having large, old amalgam fillings that do not support the remaining enamel of the tooth. Pain caused by broken tooth or some lost filling can be relieved by covering the exposed area with chewing gum. This can work with a loose filling as well, to hold it in place until meeting the dentist.