Maybe not THE sweetest, and more of a practical joke, but:

One of my co-workers in a tool and die shop was filling a large hypodermic syringe with water at the fountain, and squirting people with it. When he went on vacation, I took some plastic tubing left from a job, jammed a piece of hard plastic with a hole in it into the tube, and strung it over the ceiling tiles. A sprinkler system had just been installed, and I taped the nozzle end of this tube to the sprinkler which was over this guy's toolbox, and out of sight above the false ceiling. The other end ran to the air pipe next to my bench, down through the ceiling, and to a small water reservoir, which in turn could be pressurized with shop compressed air. There was a wall immediately behind my victim's bench, with enough room to comfortably work. When he left for vacation, the sprinkler in our supervisor's office had a drip.

The morning the victim came back to work, he found some water on his bench, and a small bucket with a littie water in it on top of his toolbox. When he asked what was going on, he was told that apparently the sprinkler crew had swapped the dripping sprinkler head in the boss's office for the one over his bench. He got on the phone, and raised Hell with maintenance. They had no idea what was going on, and weren't part of the fire system crew, anyway. He just wouldn't get into position, and finally the boss asked him for a tool of his, which was the only one in the shop. He got into the spot, and I hit the air trigger, spraying him right in the chest with a stream of water! He jumped back, hitting the wall, and we all laughed. He thought someone had dine it with a syringe, but it was pointed out all our hands were empty. I told him the sprinkler had done it. Oooooh, he was on the phone, again, mad as a wet hen. Then, he wandered back, and I got him again! God, I wish we had all the video we have now, back then. It was better than a cartoon. Of course he called maintenance again, and raked them over the coals once more. Things calmed down, after a while, and when he was in the target zone once more, another of the guys hit the trigger, and after a brief squirt, it blew air; the reservoir had been depleted. He got suspicious, then, and quickly discovered the tubing, and followed it to my bench. He had to admit he had been had, but good. I pointed out that if he'd thought about it, once those nozzles spray, they continue to, and it isn't a directed stream, but a full pattern. It broke him of squirting water on us, though. Obviously the whole shop was in the know, including our supervisor.