answer:This happens due to the physical characteristics of the can and the liquid inside. Compressed air is actually maintained under pressure primarily in the form of a liquid. However, there is some gas that evaporates “above” the liquid under normal conditions. When you release the valve, that gas escapes. In order to maintain an equilibrium, some of the liquid then boils/evaporates to (in essence) replace the released gas. That evaporation event requires energy, and the liquid draws energy (heat) from the external environment (i.e. the can), thus leading the can to become noticeably colder. Note: Before I start getting flamed for not invoking the Ideal Gas Equation, I would like to add that I am quite aware of the physical chemistry involved, but figured a simplified explanation would be more suitable.