Being a southern girl, I love this joke. Hope I don't offend anyone.  

What's the difference between northern girls and southern girls?

The northern girls say, "yes, you can" and the southern girls say, "yes, y'all can".

Must be "southern hospitality."

Southern women prided themselves on politeness, so their annswer would be, "Yes, sirs.  Y'all can."

Love the little ring that "sir" gives it.

END OF MESSAGE