Ballpoint pens have a tiny ball made of brass, steel, or tungsten carbide as their writing tip. A compartment called the ink reservoir holds the ink, and a narrow tube links the reservoir to the ball. The ball, which is fitted into a socket, turns as it rolls across the paper, carrying ink from inside the pen onto the paper. Many ballpoint pens have a plastic cap that helps prevent ink from drying out on the ball. On others, a push-button mechanism draws the pen point into the body of the pen.