Teeth are complex structures made of materials specific to them. They are made of a bone–like material dentin, which is covered by the hardest tissue in the body—enamel.[10] Teeth have different shapes to deal with different aspects of mastication employed in tearing and chewing pieces of food into smaller and smaller pieces. Incisors are used for cutting or biting off pieces of food; canines, are used for tearing, premolars and molars for chewing and grinding. Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be swallowed to make its way down the upper gastrointestinal tract to the stomach. Dental health is maintained by the salivary secretion of gingival crevical fluid.[11] The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.