Subject: Why does canned air get cold when shaken?

I have searched the web (through 9 billion "e-tailers, that would love to sell 
me some), and the local library to no end for this answer.  I asked a local 
science professor and he simply said "it's compressed air".  I am just curious 
about the reaction you get when shaking the can, or better yet, the product you 
get when you spray it upside down.  I am baffeled that no one seems to know 
what is actually in "canned air"!

Re: Why does canned air get cold when shaken?