Steel nibs made possible the development of fountain pens, and the first successful models began to appear the 1880s. In he fountain pen a slim rubber sac kept ink supplied to the nib. Although the fountain pen was a technical advance, i was not popular at first. But as public education be came more widespread, more and more people bought pens. World War I also helped boost sales. Many soldiers used fountain pens for writing letters home. By 1925 twenty-five million fountain pens a year ere being made in the United States alone