This one is really yucky but the expressions on your guests’ faces are going to be worth the trouble. Get as many diapers as you have guests. Stain each diaper with a yucky smelling food or any kind of goop that doesn’t smell so great BEFORE you hand them out to your guests. You can even pick the prank poop you get in stores.
Give each guest a diaper, don’t let on it’s already filled with yucky “poop”. Place a bowl of “poop” (left over baby food or pudding) in the center of a table and tell your guests that have to place as much poop in the diaper as they think a baby would do. Get ready with your camera; this is the prank part. When guests undo the diaper to ‘place’ the poop in their “clean” diaper they will already discover something terrible & smelly there and they are going to react as though it’s real baby poop! The guests without a baby are the one’s to really watch out for.