A town in Poland had only one cow and it stopped giving milk. The townspeople did a little research and discovered they could get a cow from Moscow for 2000 rubles - or one from Minsk for only 1000 rubles. So, naturally, --- they got the cow from Minsk.
It was a great cow: had a wonderful disposition, and gave lots of milk and lots of cream. Everybody loved it dearly. The people decided they would mate the cow and get more cows like it, and then they would never have to worry about their milk supply again.
So they got a bull and led the cow and the bull into the pasture. When the bull came in from the right to mount the cow, the cow moved to the left. When the bull moved in to mount the cow from the left, the cow moved to the right. This went on all day.
Finally, in desperation, the people decided to go ask the rabbi what to do. After all he was very wise.
They told him the story. "Rabbi, we've tried all day to mate our cow. When the bull moves in from the right the cow moves left and when the bull moves in from the left the cow moves to the right. What do we do?"
The Rabbi thought a moment and asked, "Did you buy this cow from Minsk?"
"Rabbi!" they replied as one, "You are so wise! We never said we bought the cow from Minsk. How did you know that?"
The Rabbi said, sadly, "My wife is from Minsk."