Which is the biggest city in the world? And why is such a simple question so difficult to answer? If you search on the internet for the world's biggest city, you'll find various different cities like Tokyo, Seoul, Chongqing, Shanghai etc.
Which one is regarded as the holder of the title, all depends on what you mean by "city". Most experts say that Tokyo is the world's largest metropolis, with a population of about 36 million people. But the core of the city has only eight million people living in it.
The reason it gets into the record books is that the surrounding region - which includes the country's second city Yokohama, as well as 86 other towns and cities - has become so built up that it is now one huge continuous urbanised area. Yokohama alone has a population of 3.6 million.
Defining the borders of a city is no easy task - and there is no international standard to ensure consistency. Three scholars who in 2009 compared eight different lists of top cities by population, found there were 30 "top 20 cities" in total.
Size of the city
Most experts rate Tokyo as the world's biggest city because of the size of the population in the larger urbanised area. It will probably still be the biggest in 2025, although its population is expected to scarcely increase.
Tokyo city was merged with Tokyo Prefecture in 1943 to form a "metropolitan prefecture" - it still contains the 23 wards of the old city.
Tokyo city was merged with Tokyo Prefecture in 1943 to form a "metropolitan prefecture" - it still contains the 23 wards of the old city.
And what about the cities in the country everyone is talking about - China? Its cities are growing so fast that for the first time more than half the population live in urban areas.
Professor Kam Wing Chan of the University of Washington in the United States, who has made a career out of correcting people's exaggerated claims about Chinese population statistics, explains that what China calls a municipality or city is better understood as a province.
Many of the 30 million people who are said to live in the city of Chongqing are actually agricultural workers living in a rural setting, he says.
"And if you were to travel from the downtown area to some of the peripheral areas where those 30 million live, it might take a day or two because the road conditions are not that good. So, this cannot be possibly called a city. Because when we call a place a city the general understanding is that we're talking about a commuting zone."
Professor Chan calculates that a more reasonable estimate of the urban population of Chongqing is six or seven million. The largest city in China is actually Shanghai. It is commonly thought to have a population of 20 million, but Professor Chan thinks 16 million is a better estimate.
He says everyone just loves to think China's cities are bigger than they actually are. He has even had to correct fellow experts at a world conference on global megacities of the future.
