A good practical joke is a test of wits. The wits of the perpetrator and the wits of the victim. The perpetrator tries to create a situation in which the victim is subjected to strange but harmless unpleasantness. The victim meanwhile has the chance to avoid this unpleasantness by seeing through the joke. A true master of the practical joke plays with his victim, changes his reality and leaves him hanging with the sense that something strange just happened but he doesn't know how or what. The following practical joke, if executed with skill, does just that.
What happens: Your colleague comes back from the bathroom and his monitor is frozen. He taps on the keyboard like a madman, moves his mouse back and forth and presses Ctrl-Alt-Del, but nothing happens. Even restarting the computer has no effect. After your colleague restarts you saunter over and try to help him out. You offer to crawl under the table and check if the cable is plugged in firmly. You even unplug and reinsert. When nothing works you tell your colleague he'd better get someone from IT. However when the IT comes the computer is back to normal. What happened?
How it works: This is an easy joke to set up. You can do it while your college is in the bathroom or before he comes in to work. Basically there are three ways to do it so choose the one that is most convenient. The first way is to actually switch your monitors. If you have monitors back to back this should be an easy thing to do. If you sit next to each other you can do it by simply plugging his monitor into your desktop and vice versa and lastly you can simply switch keyboards. Now when your colleague comes back he will find his screen is frozen and that there is nothing he can do not even restarting his computer (did I mention you should also switch mice?). To make sure the switch goes smoothly come in before work and make sure that the cables are long enough. You can also untangle them if necessary.
Getting it right: Now the key to success with this practical joke is a keen eye for detail. Make sure that if you switch monitors that they are actually the same also try to place the monitor in the right spot and don't disturb anything on your colleague's desk.
The next thing you should make sure of is that what your colleague sees actually looks like his desktop. So copy the background picture from his hard disk and all the icons on his desktop.
Staying cool: Now when your colleague comes back stay cool. Try to act the way you normally do. You could also go for a cup of coffee, that way when you come back your colleague will have already tried everything possible to unfreeze the screen. Here is where you come in with useful, but ultimately useless tips. It is vital your colleague doesn't try and check the cable himself. To diffuse this danger you can try intertwining cables in such a way that it's hard to define which goes where, that way he might just concentrate on the plug. If at all possible pretend to check it for him and tell him it's fine. If he tells you to unplug and reinsert make sure it is the one in your computer you're unplugging. Finally try and have your colleague leave either to get someone from IT or to get some coffee, this is where you have the chance to put things back to normal.
Spicing things up: If freezing your colleague's screen is not enough for you you can simply operate his screen from your keyboard, you can open documents and write him messages or even make him believe a virus has taken over his computer. The sky is the limit, have fun.
