Soon after brushing, a thin sticky layer forms over the surface of your teeth which contains bacteria, this is referred to as plaque. When you eat anything containing sugar, the bacteria in plaque uses it as energy and produces acid which can soften and dissolve the enamel which cover your teeth. This is how a cavity is formed. Because the enamel doesn’t contain any nerves, you won’t realize that it is happening. Only when a cavity is in the advanced stages will you feel pain. This pain will occur when you eat or drink anything particularly hot, cold, sugary or acidic. Saliva helps to wash away and neutralize this acid. It also contains the minerals that replace loss from enamel.