If your kitchen faucet has a spray handle take some clear tape and tape the spray trigger in such a way that when some one turns the faucet on, the spray handle will spray uncontrollably, mildly soaking your victim.

Tell your friend about a neat magic trick you just learned how to perform. Get a glass of water and tell your friend to place both his hands flat on the table with his palms down.

Now, tell him to put one hand on top of the other. Place the cup of water on top of his hands and walk away. If your friend moves they will get water over them.

This can also be done by telling your friend to kneel down like a table and placing the glass on their back.

Tell someone you can pin a glass of water to the wall using an ordinary drawing pin. When they refuse to believe you show them...

Get a glass of water and a pin. Hold the glass up to the wall and start to pin it up but then drop the pin. Now hold the glass in position and ask your victim to get the pin for you. When they bend down to pick it up, dump the water over their head!