An old one:
A man walks past a drunk who is sitting on the street curb, looking up into the sky , and muttering to himself. In the very point that he steps past him he realizes that the man is muttering "It can't be done. It just can't be done" over and over. It makes him pause for a moment and, as most people do when they see someone concentrating at something in a particular direction, the guy follows the drunk's line of sight up towards the clouds where he sees a billboard that says, "Drink Canada Dry".