After World War I, the US emerged as the world's biggest industrial economy. In the 1920's, the Government's economic policies brought the kind of economic growth that had been seen before. Factories became more never efficient. For example, Henry Ford had devised a way to make hundreds of cars a day instead of five or six cars a week. This was called mass production.people poured into the cities to work in the paid job . The building boom meant that buildings became so high that they scraped the sky skyscrapers.