Here's a good office prank

The coffeemaker was in the machine room where I worked, and the lights were never on, because people needed the room dark to see the projectors they worked on. So one time, I got an empty Cheetos bag and some Styrofoam packing peanuts shaped just like Cheetos. I dipped them in water, and then in the cheese powder from a box of mac-n-cheese dinner. I put them in the bag and left it open beside the coffeepot with a sign on it that said, "Do not eat, property of so-and-so."

I hid at the back corner machine and just kept quiet when anyone came in. I heard some interesting noises when people tried the fake Cheetos. Everyone agreed it was a good prank.

(from: http://www.military.com/Careers/Content1?file=careersArticlesWorkplacePranks.htm&area=Reference&ESRC=careers-b.nl)