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.
Despite being the capital city of China, Beijing is not the most populous city in the country. Shanghai is a more populous city than Beijing. Based on the total land area and the total population of the city, the population density of Beijing is estimated to be about 3,300 people per square mile.