When we drove to Utah last week, I woke up in the middle of the night kicking myself: "Why didn't we seal any marshmallows in bottles and bring them along to see what happened when we changed altitude?" So when we visited the family cabin, nearly 10,000 feet above sea level, we sealed up some marshmallows and drove them back home.
Here's a video of us opening the bottle at sea level. Notice how the marshmallows shrink when the lid pops off--that's because the air rushing in pushes them down. When the video ends, you can see that the unsealed marshmallows are a lot smaller than the sealed ones.