Have you ever been on an airplane and noticed how the packets of chips seem to have expanded more than on the ground? This is an effect of atmospheric pressure. An aircraft is pressurized to a level that is equivalent to about 8000 feet (2500 meters) altitude. Because the air is thinner at this level, it exerts less pressure on the chip packet and so the pressure inside the packet can cause it to expand.
A similar effect can be seen if you drink some water on an airplane and then return to ground level only to find that the bottle is somewhat crushed. In the photograph, Chris Terai (a graduate student at the University of Washington) is showing a bottle that he took to the high altitude Altiplano, which is 15000 feet (4500 meters) above sea level. He closed the bottle there and then returned to Arica, which is at sea level, where the excess pressure outside the bottle crushed it!!
Postcards from the Field: Climate Science from the Southeast Pacific