Teeth grind food during mastication. The exposed part of a tooth, or the crown, is covered with enamel, which is the hardest substance in the human body. The crown consists of rounded projections or cusps that facilitate the grinding process. Over time, food particles can cling and accumulate in the grooves between the cusps and between the teeth, forming a sticky plaque. Bacteria, such as streptococcus, will flourish within the plaque, eating and digesting the sugars and carbohydrates. As a result, bacteria produce acidic by-products that are harmful to the teeth. Normally, saliva will neutralize these acids but plaque provides a barrier that prevents saliva from reaching acid that is concentrated around the tooth's enamel surface.