Today I pulled a prank on my boss. I got the idea from Aimee, my super awesome roommate.
This morning I noticed the water cooler near empty, so I quickly rushed to the pet store, purchased a betta fish and some water conditioner. I drove back to the office and hid my fish.
Later, I got Robert, our warehouse manager, to assist in filling the water jug with tap water, which we then conditioned. We carefully put our betta fish in the jug and Robert calmly went to place the jug back on the water dispenser.
We giggled every time Chris looked our way, completely oblivious to the betta fish in the water cooler. Eventually he said he was going to put a bottle of urine on my desk, and I asked him how he was going to get this urine. He looked over at the cooler and filled up his bottle of water, then shrieked at the little fish.

That's the end of that story.