Long before the first LCD screen was plugged into a computer, owners had to depend upon cathode ray tube monitors or CRTs. Although capable of much greater resolution, CRTs were essentially just sophisticated TVs, and thus at risk for phosphor burn-in, which would occur if a specific image were to be left on the screen too long. To prevent this, a software program was designed to fill the screen with images or moving patterns. These were quickly dubbed “screen savers,” since a monitor with a burned-in image would become problematic. The earliest examples were in black and white, but as color CRTs became the norm, screen savers grew in sophistication. Some were free, others shareware, and still others inexpensive software that would not only prevent burn-in, but also provide interesting images and motion graphics that were pleasant to watch. Today, although LCD and other flat-screen monitors are not susceptible to burn-in (because the images are not produced by phosphors), screen savers are still a pleasant diversion while Mac and PC users are waiting, for instance, for a huge file to download.