My flights were happily uneventful. However, on the Charlotte-Barcelona leg, we were served yogurt for breakfast. The foil top of mine was extremely puffed up. This has occasionally happened to me before when the yogurt has begun to ferment and it's recommended that you don't eat them at that point. I looked for the expiration date, and found it was 05.08.15. That's actually not out of date - in Europe the dates are written day.month.year. Yogurt can in fact ferment before its proposed expiration date, but typically it doesn't go bad two months before. I peered around for a while trying to see other people's yogurts or a flight attendant to query, but everyone else nearby had already opened theirs up and there were no attendants on my side of the plane.
A seatmate on the other side asked me if I needed his help in getting a flight attendant since I was clearly looking around a lot. I said I wasn't sure - what was his yogurt foil doing? He investigated his yogurt and those of his friends, and said they were all bloated as well. I figured that while one puffy yogurt may have snuck past quality control and onto my tray, if everyone's were like that then the staff clearly must be aware and believed they were okay to eat. So I opened it and cautiously dug in while continuing to mull over the problem. Then it hit me - it's because we're on an airplane. The yogurts were packaged while on land and sealed with normal air pressure, and were then transported up with us to a lower air pressure environment so the trapped gases inside the yogurt container expanded and puffed the foil top up. It didn't occur to me at the outset because I'd only ever encountered the problem in a different context. This is one of the main reasons I like to travel: I love challenging expectations I don't even know I'm making (even when they're about something as quotidian as foil yogurt tops).