When we eat something sweet that contains sugar, some sugar does remain in our mouths. When the already existing bacteria in the mouth break down the sugar, acids are formed. This formed acid lowers the pH of the mouth. When the pH of the mouth moves to the acidic side, teeth start to decay. One has to make sure they clean their mouths properly after eating something, especially after eating something sweet. You should brush your teeth twice a day to keep them bacteria-free.