Season 8, Episode 5
"The One With Rachel's Date"
Context: Rachel is pregnant and Ross is the father, or "Baby Daddy", if you prefer. Those two are in the "off-again" phase of their relationship, and so Rachel is on the prowl for a man. Joey introduces her to one of his Days of Our Lives co-stars, Kash (Johnny Messner), and he asks her out. Rachel, who isn't obviously pregnant yet, is concerned that mentioning her pregnancy might sabotage her relationship with Kash before it has a chance to get off the ground, so she asks Joey for help finding a reasonable alternate to the truth as to why she won't be consuming alcohol on their date.
Exegesis: Hey Mormons don't drink! We don't drink alcohol, coffee or tea. We don't smoke either. It's all part of that wonderful law of health, the Word of Wisdom. Like any behaviour that deviates substantially from otherwise accepted social norms, (like total abstention form alcohol), Mormons' adherence to the Word of Wisdom has given us a reputation for clean living, and it's one of the nicer things to be known for.
There's not a lot to say bout this reference, really – Mormons are admittedly a peculiar people in the eyes of the world, and our healthy living habits are one of the more "tangible" things we are known for. Our understanding of Christ's infinite atonement is less easily grasped and articulated, but "they don't drink" is pretty easy to remember, and tends to stand out in this day and age. When I have declined offers of alcohol in the past, it's not unusual to hear the follow-up question: "Are you Mormon?" About half the time, the person has clearly asked the question as a joke, expecting the answer no, and are a little embarrassed when I answer in the affirmative. It is the joke in that sense that the writers for Friends were going for.