Well, this was an annoying prank when I was in the Navy, and watching "Lethal Weapon 2" gave me the idea: if I could make the same joke, but activated when the guy sits in the toilet? MUAHAHAHAHAHAHAHAHA!! IT'S PAYBACK TIME!!!
This devilish prank works in this way: when the victim sits in the toilet, press a switch. The switch activates a motorized device, disguised as a first aid kit in the top over the toilet, and drops a big splash of water.
BUT WAIT!!! I want to say to the victim "HEY! YOU WAS MY VICTIM!". I want to say "IN YOUR FACE!!". I want to say "YOU'RE MY B...!!!".
And for a geek, all those sentences are summarized in one single word: "BAZZINGA!!!"
So, when the water drops, immediately a "Bazzinga!" drop-sign will fall. And the poor wet human will be noticed that he has fallen for one of my classic pranks (thanks, Sheldon Cooper!!)
(NOTE: Because the victim did not give authorization for upload the video of the prank, you will see me making the demonstration of how it works. I put together the white box and the drop-sign for seeing how it works, but you can place it where you consider it will look better).