Another way to describe what happens: the liquid r-134-A boils at a certain temperature, depending on the pressure it's under. At the pressure on the high-pressure side, it condenses to a liquid as its temp falls to condenser temperature (around 100-150 degrees F). The expansion valve is simply a restriction, whcih separates the high-pressure (condenser) side form the low-pressure (evaporator) side. The boiling point of the liquid R-134-A falls dramatically at the lower pressure; it boils in the evaporator ('evaporates') at a nice, low temp, which provides the cooling to the passenger compartment by absorbing heat from the evaporator coil (the evaporator is basically being heated by the air from the passenger compartment which is blown across it's fins by the A/C blower).

This boiling of the liquid R-134-A explains why it is liquid in the can, but becomes a gas (and gets cold) when the can is punctured. The can has a good bit of pressure on it. If you heat the can enough (don't), it will boil in the can, and Kaboom.