Animal Surprise
When two people are having sex, someone runs in completely naked except for a mask (preferably some sort of animal mask) slaps whoever is on top on the ass, screaming and jumping and laughing the whole time, then runs out of the room and closes the door.
Dude, Bry was with his girl last night and I gave him an ANIMAL SURPRISE...I knew that gorilla mask would come in handy. He showed me the red mark on his ass...NICE!