This is a simple prank that takes 5 minutes to setup and will cause huge amounts of frustration for the victim. I only know how make it work on a Mac but I'm sure it would be just as simple on a Windows machine.
Basically, you're taking a screenshot of the victim's desktop and using that image as their desktop wallpaper. So what look like actual files are really just images, including the Dock. To complete the effect you need to switch the Dock preferences to Hiding On.
For many of you that will be enough to get going, but if you need any more detail just follow the steps ahead.
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Step 1: Taking a Desktop Screenshot
Firstly you need to take a screenshot of the desktop to use as the wallpaper. On a Mac this is done simply by pressing the Command/Shift/3 keys at the same time. You should hear a camera sound effect and the image will be saved to your desktop.
Step 2: Setting the Screenshot As Wallpaper
Next locate that screenshot and right click on the image file. At the bottom of the pop-up menu should be the option 'Set Desktop Picture'. Select this and the dastardly deed is nearly done.
Step 3: Setting Dock to Hiding On
Lastly you need turn Dock Hiding on. This means the real Dock will duck down out of sight leaving only the fake image Dock in view. This is handy if you need to actually use the computer at any stage because you can simply hover your mouse at the bottom of the screen and the real Dock will reappear.
Your Prank is now complete!!
Step 4: Reversing the Prank
At some stage you'll need to put things back the way they were. And the easiest way is to go to System Preferences from the Apple drop down menu in the top left corner and then select Desktop and Screensaver. Then you just select an image and set the desktop back to normal.
Hope you've enjoyed this Prank Instructable.