Fun Practical Joke! An oldie but a goody.... only works on a tile or hard surface floor...
Make a bet with a person that if you pour a puddle of water on the floor and they sit leg outward and apart around it and give them a towel you can wipe up the puddle before them....
How??????????
Well... you grab their ankles and pull them over the puddle...lol... make sure no one else that doesn't know the joke is around....