Why are pressure and volume inversely related?
See Boyles law under gas laws. Gas particles exert pressure because they collide with one another. Air contains multiple gases, each exerts it 's own pressure as it collides with another within a given space. Gravity holds air close to the earth. Air pressure decreases as altitude increases. If the space or container containg gas decreases then gases bounces againt each other and the container more frequently, causing the pressure guage to show an increase in pressure. Volume and pressure, therefore are inversely related, one goes up, the others goes down. Liquids would behave the same way.