We all know that people tend to live in cities. Recent United Nations population data clearly shows that urbanization is a growing phenomenon. In fact, the increasing concentration of population in cities has led to the emergence of mega-cities. 2015 data suggest that 30 cities already have more than 10 million inhabitants; in 1950, just 30% of the world population was living in urban areas. Today the same figure is around 56%, and this percentage is supposed to reach 68% in 2050. Consequently, two out of three inhabitants will live in a city in the next decades