Urban sprawl is the process by which a town or urban center gradually expands over time to take up more of the open land around the center. This does not always coincide with the growth of the population. A city may actually shrink in terms of population even as it grows outward, thus creating many abandoned buildings within the city limits as new ones are constructed on the outskirts. There are many reasons why this happens, but the main reason is that the price for empty land tends to be less than the price for land with a building already on it. Businesses and homeowners who would just have torn down existing buildings anyway are often better off, from a financial standpoint, to buy the empty land and then construct the buildings on it afterwards. Cities often spread toward the cheapest land, in the opposite direction of lakes and rivers.