A Jewish man decides to convert to Catholicism. So, he goes through the whole rigmarole of conversion, and now he's a Catholic. But, the very next day, when he wakes up, he puts on his tallis and tefillin and starts davening.
His wife takes a look at him and says, "Sam! What are you doing? You're not Jewish anymore!"
Sam smacks himself upon the head and says, "Oy! Goyische kopf!"