The humble, and sometimes not so humble, ball point pen is the most widly used writing instrument worldwide.  In its simpliest form the ball point is a tube filled with a thick oil based ink.  On the writing end there is a tiny usually steel ball in a socket.  As the pen moves the ball rolls and as it rolls it picks up ink on one side and applies it to paper on the other.  Our ball points use widely available and replaceable ink supplies so they can last a lifetime.  The writing technology is common, the pens are anything but common.