Note the shape of the bottle: It’s a bit thinner and taller than the 20oz bottles common in the US. On our drive to the airport Saturday night, we stopped at a gas station in order to get coffee to keep David awake and all of us alive, and I got some 7 Up for myself. It tasted slightly different from what I remember from the States, though I don’t know when I last had any actual 7 Up. I finished it in the Morelia airport while waiting at the gate, after the baggage-searcher said I should drink it before boarding the flight (not that it would really matter). I tossed the empty bottle in my backpack as a souvenir. (I had considered swiping the Spanish Coke can from the flight in, to complement the Swedish one my dad brought home from a business trip.) When I got home and unpacked, I found something curious.

The bottle was now squished, having done nothing but taken two airplane flights with me back to Rochester. The top has been on tight ever since Mexico, and I still haven’t opened it even now, perhaps in case I ever need a sample of Mexican air (not to mention a nice example of physics in action). I pondered why this might have happened for a few minutes, and then asked my dad what he thought. He asked one question, and I suddenly realized what was going on. I wonder, how many of my readers are astute enough to figure it out? (I have no doubt that my physics and engineering friends at RIT will have no trouble with this.)