Ink. Ballpoint pens employ a thick oil-based ink, whereas rollerball pens use a water-based ink that is more comparable to fountain pen ink. Water-based ink is more fluid and often delivers a more comfortable writing experience. Both types of pen use an ink cartridge or reservoir that holds the ink. Ballpoints are commonly used for drafting purposes because they produce very fine lines on paper. They are also useful for writing by hand instead of typing because their large ink drop size makes it possible to fill in large areas quickly. Finally, ballpoints are ideal for doodling and drawing because they can be easily controlled with slight movements of the wrist. Dot matrix printers work by using an inkjet printhead that has several nozzles attached to it. As the printhead moves across the page, it drops ink onto the paper. The printer controls which parts of the image get printed in black and which parts are left blank by controlling how much ink comes out of each nozzle. The advantage of dot matrix printing over line printing is that you can put any picture into the printer. With line printers, only simple images like letters and charts can be printed because the printer needs to know where one line ends and another begins. With dot matrix printing, the printhead can sweep across the page depositing ink wherever there's a hole in the image, so complicated pictures can be printed too.