My nephew, who's 10, got his parents soooo good
You know the little sprayer that's next to the kitchen faucet? Hopefully your faucet/sink is set up that way. Well, he put a rubber band tightly around the sprayer so that it was "on" and kept it in its normal position, aimed outward -- basically aimed at anyone who was standing at the sink. Everyone who walked up and turned on the sink got a small blast of water.
No one got hurt or soaked ... just a little spray of water and a big surprise!
I'll tell you, the kid's brilliant. Not only did he think this one up all by himself and get both parents with it at different times, when he was around five, I asked him what he wanted to be when he grew up. He said he wanted to work for NASA (we're close by). I said, "Oh, yeah? You want to go up in space?" He said, "No. I want to build the space ships."