A man walked into a bar and saw two beautiful blondes at one of the booths.
He walked over and introduced himself and they started chatting.
A little while later he invited them back to his place, for a little "party"...
As they were heading out the door, he asked them "You two are beautiful, are you sisters?"
One of the blondes giggled and said "Why, no, we are not even Catholic..."
Then my customer said "Get it... they are not Catholic, like Sisters, you know NUNS!"