The dentin of teeth will be exposed if one is suffering from chronic:

If the enamel or cementum gets worn down or if the gum line has receded, then the dentin becomes exposed. "Cavities, cracked teeth, gum recession, enamel and root erosion all cause dentin to be exposed," Culotta-Norton said. "Dentin is connected to the nerve that triggers pain in sensitive teeth."

one would be wind another would be water density