What's a good practical joke to pull on a co-worker?

Well, I've done some things:

Snuck into the ladies room with some blue food coloring and poured it into each of the toilets (no tanks). One of the women ran to the supervisor that THERE'S BLUE WATER COMING UP IN THE TOILETS!

I poured some water under the back of our fork lift. The driver went nuts trying to figure out where the water was coming from. I did this 3 times that day.

I put a small piece of tiny bubble wrap under the back tire away from where the driver got on the fork lift. Sounded like fireworks!

I turned off the propane gas bottle on the forklift. The driver thought it was empty, but when he went to take it off, it was still full.

We have 'critters' coming into our warehouse (twice, raccoons got into offices from the ceiling); foxes, 'possums, raccoons, cats. I can imitate a cat pretty good and a few times I had some women looking for the cat that I had imitated.

I put an OUT OF ORDER/NO WATER sign on the women's rest room. They had to use the men's.

I left a note on a supervisor's desk to call a MR. LYON and the phone number was to the Norfolk Zoo.

One of the female supervisors (long story why) will not let anyone even HEAD towards the door until exactly 3 p.m. The 'official' time is on the microwave. When no one was around, I set the time on the microwave 3 minutes ahead as well as the 4 clocks.

I superglued a quarter to the floor in front of the soda machine.

When I was in the Navy, on 2 of my ships, one of my shipmates and I would be standing in the chow line and would start rumors. On one of the repair ships I was on, we started one that we were going to make a cruise to the Mediterranean. We were close. A month later we got orders to go to Diego Garcia (an atoll in the Indian Ocean), and had to go THROUGH the Med.

On the Flag Ship for the Middle East Forces, a shipmate bought a deck of cards (Italian) and we pretended to be playing a game. One guy actually stood there taking notes on how we were playing!

In boot camp, we had a GOMER PYLE guy. He was sitting there writing a letter and I walked up to ask who he was writing it to. He said it was to his sister. I asked if she was going out for track or cross country that year. He looked up real sad and said, "Andy, you KNOW my sister is paralyzed and is in a wheel chair!" Everyone was getting down on me until Gomer and I started laughing. GOTCHA!

We used to do stuff like that to other companies. Two of my partners and I were in the base theater (in boot camp). Me and Bell went up to the snack bar and ran into a guy we knew from another company. We told him that Womer's mom always tried to make Christmas cookies but always burned them. We told him to go down and ask Womer if his mom was going to make cookies that Christmas. So, he did. Womer looked up at him and said, "That's not funny. My mother died in a fire last Christmas." Needless to say, me and Bell made ourselves scarce! No, Womer's mom DID NOT die in a fire. She was alive and well. Boot camp was in the winter of 1965.

That SO totally depends on the co-worker, and the best ones I've seen have been specific to that co-worker.

Example #1 - have someone who the co-worker doesn't know call them up about their mower repair, giving some huge estimate.

Example #2 - take the mouthpiece out of YOUR phone. As they are leaving a voicemail for someone, show them the mouthpiece - "The phone works better if you have one of these".

Tell them you just over heard a conversation between your boss and the CEO about planned layoffs in that department.

Poop at work, put it in a DHL box and put it on their desk.

Sometimes it may backfire on you. Be careful. Not everyone has the sense of humor that you think they do.

Work place is not the same as your dorm.

None, it will lead to trouble.