Some cities grow very large because of two important reasons. First, there may be important natural resources like wood, gas, oil, rivers or harbors near or in the city. Natural resources like wood or oil can be brought to the city and made into products to sell. Other resources, like rivers or harbors help to send the city's products to other places to be sold. Second, the city may be located in a place where roads and rivers come together. This makes these cities good places to buy and sell goods. Houston is a city that grew large because it has two important natural resources. They are oil and a good harbor. The oil can be brought to Houston, made into different products, and shipped out of the harbor to other parts of the world. Chicago is a city that grew very large because of its location at a place where roads, railways, and airways meet. In Chicago, goods can be brought together from all over the country and bought and sold. Then the goods can be loaded into trucks, trains or planes and sent to wherever they are needed. Because of Chicago's location, many people live and work there. Some cities grow large because of ______.