One day, 75 year old Shmuel, a very Orthodox man, suddenly says to his wife, “Rivkah, I’ve been thinking about this for 25 years already and I’ve finally decided to become a Christian.”
“Shmuel,” says Rivkah, “do what you think is best. You’ve obviously thought long enough about it already.”
So Shmuel goes through and completes the process of conversion. The very next morning, he returns to his corner of the kitchen and starts to lay tefilin. Just then, Rivkah enters the kitchen and sees what he’s doing.
“Shmuel,” she says, “vus machst du? Yesterday you became a Christian and today you’re laying tefilin? It doesn’t make any sense!”
Shmuel instantly stops what he’s doing, slaps himself on his forehead and says, “Goyishe kup!”