The outer layer of your teeth, the enamel is the hardest substance in your body, but under the right conditions involving bacteria, sugars and a sticky film called plaque, tooth decay can begin in the enamel, then spread to deeper layers of the tooth. One of the most common places for decay to start is on the crown or visible part of a molar tooth. The crown, which is covered with enamel, has pointed or rounded projections called cusps pits, furrows and grooves between the cusps may be quite deep and difficult to clean with a toothbrush. The thick, slippery part of saliva that helps food slide down your oesophagus, called mucin, may cling in these grooves and pits bacteria and food debris stick to the mucin, gradually, creating a thick, concentrated film called plaque. Some of the bacteria, including species of streptococcus and Lactobacillus. Produce acids when they digest sugar, their favourite food. The plaque keeps the acid in contact with the enamel, where it gradually dissolves the minerals that make the enamel hard. A cavity starts to form. There are no living cells in the enamel, so usually a cavity that is limited to the enamel layer causes no pain. A tissue called dentin lies just below the enamel at the crown.