Ah yes, it’s a prank as old as time, the “Pudding Surprise.” It’s quite simple really: step 1) find a parent friend of yours who can’t look at a dirty diaper without screaming; step 2) while your buddy’s not in the room, fill his/her baby’s diaper with pudding; step 3) show him/her the “mess” his/her kid made; and step 4) let hilarity ensue.
It’s fantastic up to the point where the screamer traps himself in a corner and grabs an ash tray to defend himself against the diaper. Then it gets amazing.