Dental decay, also known as dental caries, begins first inside the tooth. A white spot appears on the enamel where the tooth has started to weaken inside. At this stage, the tooth can repair the weakened area with the help of fluoride and minerals in saliva.
Beijing, also Peking, is the capital of the People's Republic of China and one of the most populous cities in the world. Its population in 2013 was 21,150,000. The city proper is the 3rd largest in the world.
These districts, which contain nearly all of the remaining hutong (see photo above) residences grew only 2.2 percent. At 61,000 people per square mile (23,500 people per square kilometer), inner Beijing approaches the population density of Manhattan or the Ville de Paris.