Pressure

This is what it looks like after you open a plastic bottle at 7,000 feet and leave it tightly closed until you reach sea level. Crossing the Rockies is a surprisingly noisy endeavor - bottles crackle and pop, cans flex and ping, and objects of mystery emit occasional bongs, thumps, and sighs. Add the high rpms of a laboring diesel and an engine fan that comes on with a deafening roar when the water temp hits 200, and sleeping becomes iffy at best.

Oh, and it's important to remember this pesky physics stuff before you begin opening bottles of mustard for a sandwich. Just sayin'.