It requires that you have a certain closeness of relationship in order to comment on the fact that someone else farted. It’s not something one does in polite company or in a professional setting. In such settings, the onus is on the farter to either a) excuse him or herself or b) ignore it and hope nobody notices. So why is it considered socially acceptable to comment when someone else yawns?
I’ve spent the past decade teaching myself to suppress my yawns for this very reason. (I was already pretty good at suppressing farts and, per discussion above, nobody ever comments on this anyway). Suppressing yawns became immeasurably easier when I finished residency, but one does slip out on rare occasion. I was up watching the late coverage of the Olympics last night. I hadn’t gotten that much sleep, so I decided to let out a quick yawn – no sound, hand over mouth, jaw only half way open – after placing my sandwich order for lunch. “Oh, don’t do that,” the middle-aged woman behind the counter scolded. “I’ve still got hours to go.”
You’ve got hours to go? What does that have to do with anything? Yes, I yawned. Yes, it’s a signal that I’m more tired than usual. And no, it’s really none of your business! Farting, in a way, is much more everyone’s business because of the sound and the smell it can create. But a yawn? How about if we make a deal, Von’s deli worker: you don’t comment on the fact that I yawned and I won’t comment on the fact that you have bags under your eyes, need to loose about 20 lbs and, truth be told, really didn’t even do such a great job on my sandwich?