Quite a bit of my travels involve changes in altitude. Going from sea level to over 7000 feet makes a big difference. As you go higher, the air pressure is less. This can do bad things to the stuff in your suitcase, such as causing shampoo to squirt all over your clothes. It would be nice to fly all of you up into the mountains to do this experiment, but I can't quite afford that right now. Instead, we will cause a change in air pressure by heating and cooling it.