Very simple joke if your kitchen sink has a separate sprayer nozzle: tape down the nozzle button/lever, and point it towards the counter edge. The really funny part about it is that most people will turn on the faucet more than once to figure out what the problem is, and why their entire front side is suddely wet.
One of my college roommates used to get me with that one monthly. He'd somehow instinctively know when I stopped checking the sprayer. I still compulsively check spray nozzles, or turn them towards the wall if I'm in an unfamiliar kitchen.
