Play the mission where you have to escort a friendly and get information from villagers. Once you get the information from the second person, silently kill all terrorists in that village. Then, sneak up on the outhouse and sit in front of it. After about thirty seconds you should hear a terrorist scream "Oh God -- how can you be constipated and have diarrhea at the same time?". Additionally, if you wait long enough at the overview briefing of any mission, Fireteam will begin to make fun of the mission or those involved.