Really confuse your friend by taking a screenshot of their desktop, and setting that image as their wallpaper. The most evil among you can then remove all their documents and folders from their desktop (hide them in a separate folder) and watch as they click on folders and documents and complain that they are not opening. Hilarious!
How to take a screen shot:There are lots of ways to
take a screen shot on a Mac, as we discuss in this
article, however, the best way to grab a grab of their screen is to select shift-cmd-3. If you are going to use this as wallpaper, drop the image into the Pictures folder on their Mac.
How to change wallpaper: The quickest way is to right-click (ctrl-click) on the desktop, choose: Change Desktop Background, then go to Folders > Pictures and select the wallpaper of deception.