Too funny. From the awesomeology blog:

World’s Best Office Prank First buy lots of paper cups (it’s important to get paper, not plastic or foam, you’ll see why in a minute).And I mean lots. Dozens and dozens.Arrange them over every possible inch of space on the victim’s desk.Then staple them all together as close to the rim as you can, each cup to the ones on each side, so they are all attached. (This is why you need paper cups, foam or plastic split or disintegrate.) Wherever one cup touches another at the rim, staple.Next, fill each cup as full as you can possible get it – surface tension full – with water.Finally, retire to a safe distance, wait, watch and enjoy.