this is a great prank for mac users you take a picture of the application they use most of the time and then set it as there background
Step 1: how to do it
to take a picture of your mac screen apple command, shift, and 3 at the same time this takes a picture of whatever is on the screen.
next all you do is set it as the background, to do that you go to system preferences and hit desktop & screen saver. after that hit choose folder, desktop and the picture of the screen saver. it will keep them confused for hours!