If you are itching to pull a harmless but hilarious prank on anyone, look into this message mug. On the bottom of the normal-looking white cup, it has the written message “I spat in this mug” that will be sure to aggravate confused reactions.
The funniest part will be the fact that the person will have no clue what is going on until they've finished the entire drink. It is a prank that is slow and enjoyable to watch. When they see the message, it will be too late to turn back. As long as you don't say anything, the victim will really have no clue and wonder all day what exactly was in the drink.