How Ballpoint Pens Work

The key to a ballpoint pen is, of course, the ball. This ball acts as a buffer between the material you're writing on and the quick-drying ink inside the pen. The ball rotates freely and rolls out the ink as it is continuously fed from the ink reservoir (usually a narrow plastic tube filled with ink).
The ball is kept in place -- between the ink reservoir and the paper -- by a socket; and while it is in tight, it still has enough space to roll around as you write. As the pen moves across the paper, the ball turns and gravity forces the ink down the reservoir and onto the ball, where it is transferred onto the paper. It's this rolling mechanism that allows the ink to flow onto the top of the ball and roll onto the paper you're writing on, while at the same time sealing the ink from the air so it does not dry in the reservoir.

:The ball fits into the socket with just enough space to move freely. The size of a ballpoint pen's line is determined by the width of the ballpoint. A "point five millimeter" (0.5 mm) pen has a ball that will produce a line that is 0.5-mm wide, and a "point seven millimeter" pen (0.7 mm) has a ball that will produce a 0.7-mm line. Ballpoints come as tiny as "point one millimeter" wide ("ultra fine")