Up to this point, the Apple Macintosh had a very different operating system from the dominant Microsoft DOS: its GUI. It allowed users to click on items they wanted to open instead of typing commands. Of the GUI, Gates said that “to create a new standard, you need something that isn't just a little different; you need something that's really new and that really captures people's imagination – and the Macintosh, of all the machines I've ever seen, is the only one that meets that standard” (Watson 159). Of course, after recognizing that Apple's operating system was more user-friendly and attractive than Microsoft's DOS, Gates created Microsoft Windows, which also used the GUI system. Apple sued Microsoft for copying their idea, which forced Gates to have to license the right to make Windows look like the Mac operating system. Windows didn't catch on right away, probably because most software programs were still coming written to work with MS-DOS or Macintosh. It took several versions for Windows to catch on. Despite the apparent failure of Windows, Gates became a billionaire in 1986, when he was 31 years old. In 1987, Microsoft became “the number one seller of computer software” (Sherman 31). It wasn't until 1990, when Windows 3.0 was launched and Microsoft spent $10 million on advertising, that Microsoft finally saw its greatest success with its operating system (Sherman