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.
The population of Seoul in 2016 is estimated at 10.29 million, although this is just the population of the Special City, which has a density of about 17,000 people per square kilometer (45,000/square mile). The sprawling metropolitan area is much larger at 25.6 million.