An unopened bag of chips expanding as one drives up a mountain is a good example of how volume

An unopened bag of chips expanding as one drives up a mountain is a good example of how volume depends on the pressure. In fact, in the mountains the atmospheric pressure (that acts outside the bag) is lower than the pressure at sea level (because of the higher altitude). Boyle's law states that for a perfect gas, if the temperature is constant the product between the pressure p and the volume V of the gas is constant:

Therefore, if p is lower, the volume V increases, and this is why the bag expands.