Its always fun to pull over a fun and harmless joke on a friend or co-worker. I love this one because it can drive even the experienced person nuts and yet its so easy. Plus it causes no harm, its just a harmless prank.

First you have to have access to their computer. Take a screen shot of their desktop background. Screen shots are easy to do and simple click on print screen > open paint and hit "ctrl + V"

when you do this it will paste in your screen shot of their desktop - here is mine: