Make your victim think you just made him a delicious grilled cheese sandwich, but in reality its actually a sweet dessert. For the bread just use pound cake bread and for the cheese, use white frosting that you color yellow with food coloring.
Pin Holes in Cup
Use a tiny pin to make little pin holes in a plastic or styrofoam cup. Then offer the cup with a drink in it to your unaware victim. They will have dribbles of the liquid all over their shirt while drinking the beverage.
