Currently, on Earth, there are 206 countries and territories and the area of ​​the Earth is about 510,100,000 km². We have seen how big the Earth is but population explosion is still one of the big problems in developed countries. I can take for example, the population density in the Mauritania desert which is 3 people per square kilometer but in Hong Kong city it is more than 6000 people per square kilometer even though the desert is 10 times bigger than the city. The reason for the above problem is due to the high number of people moving from the countryside to the city and partly due to the harsh climate and weather of the desert that makes it impossible for people to live there. So countries need to take measures to overcome the problem of population explosion otherwise even we will not know what bad things will happen in the future.