Sprawl is a term planners use to describe the way that the suburbs spread out around a city. The reason they do this is that cities used to be very tightly packed. Originally they were like this because there were far fewer people than today, and because most people had to walk to where they wanted to get to, and even the wealthy few had to go by cart. Nobody much had gardens, and most people therefore lived in the sort of city you find in the middle of cities today.

Then the railway was invented, and this made it much easier for people to travel longer distances - so long as they were close to a railway station. So little villages in the countryside near to stations began to be attractive, and new houses were built there. These were actually the first suburbs, although we wouldn't think of them in that way today because they are now far inside the modern city.

Then motor transport was invented. At first this meant electric trams and motor coaches. So new houses were built along these public transport lines. But after World War 2 it meant private motor cars. Now, for the first time, people who had a car could choose to live wherever they wanted. They did not need to use trains and buses. So builders could use up all the land between the main roads and give winding little roads with detached houses and gardens. Now these took up far more room than any of the previous kinds of houses. Not only did they have a garden, but many had a garage. So cities began to spread out - sprawl - very quickly. Those are the places we see as suburbs today.