Picture of computer screen prank
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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

Picture of how to do it
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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!