A farmer noticed that his cow had turned cross-eyed, and wondered if that meant anything bad. So he called in a veterinarian, who asked for a rubber hose. The farmer brought one, and the vet stuck one end up the cow's ass and blew hard on the other end. Sure enough, the cow's eyes straightened out. The farmer paid the vet and thanked him for showing the cure.
In a week or so, the cow was cross-eyed again. The farmer tried the same treatment, but it didn't work. So he called the vet again, repeated the treatment, and said "Show me what I did wrong". The vet took the hose out and put the other end up the cow's ass. "What good will that do?" "Did you expect me to use the end you had had in your mouth?"